Gfunk6 has it pretty spot on. As someone entering a "fast-track" program in IM/Onc next year I've done a bunch of research on this program. Only thing I would change is that the post-doc is usually 3 years. For me, going into Med Onc (no Heme if I can avoid it) I'll be doing 2y of IM, 1y of Onc and then a 3y post-doc. The reason for the 3 years of post-doc is that, in order to fulfill the ABIM requirements for clinical exposure time, you need those 3 years of time doing continuity clinic during your post-doc. ABIM requires 20 months of clinical contact during the 2 years. This can include all inpt. rotations (including specialty wards if your program has them) and ambulatory rotations. It does NOT include consult months or research. Which essentially means that you'll only have 1-2 mos per year (depending on how your program does vacations) to do consult electives, the rest of the time, you'll be on the floor/unit.
There are some obvious downsides. The IM stuff is not so much shortened as it is compressed. Basically what you're getting rid of is the 8-10 months of electives/research months that most IM residents do during PGY2/3. For me, that's fine since I'm not that interested in spending time doing Derm and Rheum rotations. You will also have no time to do research doing the first 3+ years, until you're finished w/ your clinical work.
The total time of the program depends on what specialty you choose. Anything that's usually a 2yr fellowship becomes 1y, most 3yr fellowships become 18 mos and Cards (I think) is 2yr, all followed by the post-doc.
Now that I spent all this time typing, you can find the official dirt at:
http://www.abim.org/cert/respath_pp.shtm
Leaving IM for a minute, I can think of 3 other specialties where research is easy to do (there are probably a lot more that I don't know of). Most academic Gen Surg programs now require 2y of research. This goes a long way towards explaining why upper level surgery residents are so miserable b/c their 5y program is now 7. Pathology is another place where there's generally room to do significant research within the 4 years. A classmate of mine is entering Columbia's AP/Neuropath/Research program. On paper, it's 2y AP, 1y NP and 1y Research. In reality, in order to get enough good research to have a K award and be marketable as a junior faculty or above you need to do a couple more years of research. I have a friend who just finished this with about 18 extra months of research. Finally, most academic Derm programs have a 2+2 option where, after prelim year, instead of doing 3y of clinical derm (with 6-12 mos research depending on the program), you do 2y of clinical and 2 of research. Then you bag the whole thing and do Botox on Park Ave. for $1.2 million a year. I have another classmate headed down that road this year.
Hope this was helpful. Good luck with the road ahead.
BE
at the Botox on Park Ave 
