With passing (including "just passing") scores on the board exams (step 1 and step 2), and good letters of recommendation; together with a confident, articulate performance on interview, IM has always been attainable with most people matching in one of their top 5 choices provided that they apply to mid-tier programs as well.
I have no idea whether the applicant pool has increased for the 2004 match cycle.
My IM PD told me a few months ago that the number of applicants to IM has been decreasing every year, particularly the number of US grads. I haven't heard anything specific about this year's numbers, but with the new 80 hr work week, I suspect that this year will continue the trend with even fewer applicants in IM since people will be more willing to consider the surgical subspecialties if that's what they really want to do with the more humane training hours. ~85% of US med students get into one of their top three choices in residency programs, I suspect that the percentage for IM matching into the top 5 is pretty close to 95-100% for US grads. When I say top 5 though, I am referring to the top 5 programs who invited you for an interview. If you are talking about top 5 programs in the nation that you happen to like, that's still going to depend on your strengths as an applicant, your letters, and your numbers. The top 5 IM programs (Harvard, Hopkins, etc) mainly interview AOA applicants or applicants with things on their CV that make them "unique".
I agree with Kalel. IM is a buyers market. Almost everyone gets their first or second choice. IM is THE largest specialty, with typically 40-50% of all categorical positions being filled by IMGs. Having said that, virtually all of the good programs (top 25 or so) will fill with almost exclusively US grads.
A better question might be- How many people in your class are going into IM. For me, in my class of about 210, we have only 25 (roughly 10%), give or take a few, that are going into IM. I have heard that UTSW has about 60-70 students applying to IM. It varies from school to school and medical schools with strong IM programs typically have more students applying to IM (ie UTSW).
When I interviewed with the IM chair at Baylor in Houston, he half-jokingly told me that if it wasn't for cards and GI, there wouldn't be anyone applying to IM.