I'm an NYU student and I can tell you from my own experience that getting interviews in the south was not much of a problem. I got interviewed everywhere I wanted except Duke and ended up matching at my top choice, UVA. I don't think there's any bias against us being from the north (though I do think the reverse occurs)
In terms of our match list this year, I matched at UVA and another person matched in Miami. That was it as far as people coming south of the mason-dixon line for IM. That, however, is a reflection of the preferences of NYU students. It is remarkable how they apply to the Northeast, Midwest, and California and entirely skip the programs in the Southeast, even though all of the programs there are very good. (in particular a lot of them go to Georgetown and GW and won't travel two hours more to look at UVA) That being said, though, I'm sure that any one of our good students would have gotten interviews whereever they wanted in the southeast.
The other thing is none of the students here have much knowledge about the southern programs. Two programs I would have applied to, could I do it over again, would be UNC and UAB; however I really didn't know much about either of these programs until I started interviewing in the south and talked to other interviewees. Of course, the faculty advisors will all tell you that programs like vanderbilt, UNC, UVA, etc are all very good, but you won't be able to get a student's perspective, since we really don't send students to the south. For instance, if you wanted to know whether our graduates were happy at a place like Vanderbilt, nobody will really be able to answer that question for you. If you wanted to know the same about Cornell, Sinai, Columbia, etc, then the advisor's here can set you up with a number of contacts.
One more thing to note is that city life is not for everybody. Most of the students at NYU can't leave a big city, but I ended up getting sick of it and wanting more of a smaller town to live in. Keep that in mind--living in NYC is definitely different from living in suburbia.
Hope that answers your question.