I'm a U.S.-IMG (actually senior) and I know of several people in my class who got '"pre-match" offers. These are only offered to DO students and IMGs, considered (as was pointed out) "independent applicants". You do not have to be a previously trained IMG, etc.
Trust me, this is not a huge advantage. Most of the programs that offer pre-match spots have had problems historically filling in the match, are lower-tier programs, etc. It is tempting when you get an offer ("a bird in the hand..."), but it does defeat the purpose of the match. The only way I think it would truly be prudent to take a spot given such an offer is if you have serious concerns that you will not match (i.e., few interviews, not-very-competitive scores/application, etc.). Otherwise, it's worth it to stay in the Match, IMHO, and see where you fall out.
So, it does suck that U.S. seniors are not allowed to accept out-of-match offers, but in the end you'll probably have better opportunities to interview at programs that won't even consider a DO or IMG application.
Good luck to everyone. Just remember, the program you end up at is the best program for you.
-Skip