Hmmm... That's odd... I really haven't heard of anyone that was accepted, but automatically put in the decel program... Perhaps you were admitted into one of the following programs? (Call your admissions officer to clarify)
Academic Enhancement Program: Excellently detailed here: http://www.valuemd.com/ftopic7923.html There are a lot of people placed in this program, so this is most likely what you have been accepted into.
Audit Program: You take the full load of first term classes. If you achieve below a certain GPA, you are dimissed. If you achieve above the cutoff GPA, then you are allowed to continue in medical school. If and only if you achieve a 4.0 (A's in all classes), then you don't have to repeat 1st term, and are allowed to continue to the 2nd term without repeating.
If this is something new that they are doing, and you are indeed admitted to the Alternate (Decel) Program, then your 1st term will be split into 2 terms. You will take one 6-credit class (either Anatomy or Biochem) along with one of the smaller classes (Histology=4 credits or Embryo=2credits). Then the next term, you finish the 1st term by taking the 2 classes you did not take previously. You also will take some classes that only meet occasionaly, such as PDP (Professional Development Program) and Clinical Skills.
You should have enough time to study for USMLE Step I, as hundreds of August entrants do each year.
And just to clarify, it will take 2.5 years to complete Basic Sciences... Then another 2 years of Clinicals, and then 3-7+ years of residency... That's when you'll actually be finished. :-)