I was recently reading about a Boy Scout that created a nuclear reactor from scratch, and I was wondering if it's legal to acquire radioactive materials from a smoke detector. I'm aware that he was arrested for stealing some from a nearby hotel, but if you were to buy smoke detectors for that exact purpose, is it legal to take them apart and handle radioactive material? Would you need a permit of some sort, or maybe even a degree?
You would probably need a permit.
The United States Nuclear Regulatory Commission doesn't directly address this particular issue that I could see, but based on my reading of some parts of the Code of Federal Regulations which they link on their site you would probably need a permit. I do not know if there is any case law or other federal regulations on this issue, nor do I know if any state law might apply.
The common form of radioactive material in a smoke detector in the US is americium-241. You do not require a license to own a smoke detector containing not more than 1 microcurie of americium-241 as long as it is in "the form of a foil and designed to protect life and property from fires." 10 CFR §30.15(a)(7). However, removing the radioactive foil from the smoke detector would probably take you out of §30.15 compliance, as it would no longer be fulfilling the protective function. If you are no longer compliant with §30.15, you may fall under 10 CFR §31.8(c)(1), which grants to licensed persons a general license to acquire not more than 5 microcuries of americium-241 at any one location of storage or use in the form of calibration or reference sources.
The safest course of action would probably be to speak to an attorney and get a license for the material first.
I'm not a lawyer. This is not legal advice. I am not an expert in this field of law. This answer is likely incomplete, and an actual attorney who works in this area would hopefully be able to give you a better answer. I hope this is up to the standards of this forum.
 Though you could potentially make a case that even though it has been removed, as long as it is still a foil which was DESIGNED to protect life and property, it would fall into that category still. It would be a bit of a stretch, though, in my opinion.
 It is not clear to me whether this includes smoke detectors. The technical details there are a bit beyond me. However, the two radioactive materials mentioned here are the most common ones in smoke detectors, so I suspect it has some relevance.
Background of the question
You are talking of David Hahn, aka the "Radioactive Scout". In 1995 he was 17 when he impersonated a licensed person to acquire radioactive material. But he never got enough material to build a reactor: he built a neutron source from a block of lead, into which he had stuffed lots of somewhat purified radioactive material. He also conducted radioactive experiments without a licensed lab - which is decidedly illegal.
Now, he realized that his neutron source was starting to generate dangerous radiation and dismantled his setup - which was what got FBI and consorts to the table. They cleaned up what they found - while his mother already had disposed of most of the experiments via the normal garbage - also decidedly illegal.
Since he was 17 and his mother did commit suicide during the year after the events and before her disposing of waste was discovered, neither was prosecuted at that point.
Someone alleged in 2007, he was again amessing radioactive material, the FBI investigated but found the tip not enough to warrant more investigation after standing in front of the door with a radioactivity detector and talking to him on the phone, as a report shows: "No immediate threat existed with regards to allegations that Hahn possessed a nuclear reactor within his residence."
Later in 2007 he was found guilty of stealing smoke detectors, which some people alleged he planned to extract Americium from. This claim was never at trial, so is to be taken with a grain of salt. The charge was Larceny. Not attempt of obtaining NCBR-Material illegally.
To work with materials of radioactive means, you need to be compliant with NCR rules and acquire the needed license. There are companies that have the license to dismantle and extract the radioactive material from smoke detectors - and USPS provides a list of companies that take them and are allowed to - because they make them in the first place and may handle it because of that license.
(a) Except for persons who apply byproduct material to, or persons who incorporate byproduct material into, the following products, or persons who initially transfer for sale or distribution the following products containing byproduct material, any person is exempt from the requirements for a license set forth in section 81 of the Act and from the regulations in parts 20 and 30 through 36 and 39 of this chapter to the extent that such person receives, possesses, uses, transfers, owns, or acquires the following products:
(7) Ionization chamber smoke detectors containing not more than 1 microcurie (μCi) of americium-241 per detector in the form of a foil and designed to protect life and property from fires.
(b) Any person who desires to apply byproduct material to, or to incorporate byproduct material into, the products exempted in paragraph (a) of this section, or who desires to initially transfer for sale or distribution such products containing byproduct material, should apply for a specific license pursuant to § 32.14 of this chapter, which license states that the product may be distributed by the licensee to persons exempt from the regulations pursuant to paragraph (a) of this section.
However, NCR also found that it would need 10-million used smoke detectors in normal trash to become a problem - but that assumes them to be full units, not someone ripping them apart.
Yes, you can buy radioactive material as such legally and without a permit.
You can even buy it online. United Nuclear Scientific Supplies sells it in US.
This amount is tiny though. You would need to order maybe millions of unit to kill one single person. Add to it that it has a half life of 140 days, which implies you'll need to gather it all in a rather short period of time.