I was an intern for a Federal Law Enf Agency for about 3.5 months, it was not with the ATF, however maybe I can shed some light or at least help you with your questions, I worked on a task force which brought me into contact with every agency, including the DCIS (Def Criminal Investigative Services) Now Im back in college as it was last Sum/fall and I got credit, but I did spend 50 hours a week and have some good info on what its like to be an intern for a Federal LEO. IM or email me with any questions I MIGHT be able to answer. I dont know it all obviously, but I did get exposed to alot. I will say this, my internship was not a "Intern, go get the coffee" type of internship, infact I never got coffee once, just a few line up print outs or DL info. Oh yea, and there not all JBT's, just to try and dispell the myth a bit.... they two guys I was attached with were really out there to help people. Especially when it was a Child Exploitation case.
The process is lengthy and somewhat discouraging with all the paperwork essays etc, but it is worth it if you really want to see if it is something you might want to do in the future.