Elaine,
Unfortunately, it seems as if a lot of people get that offer. I did (sculpture MFA) and my friend, who got rejected as a photo candidate last year, also got the offer. It basically is an opportunity to build your experience/portfolio to increase your chances of getting into an MFA program in the following year. It's ususally a one-year program (don't think that's variable), but I have heard that it actually does increase your chances significantly (I heard MICA MFA students haveing a panel discussion about it and tons of them were prior Post-Bac students.
On another note, where are my sculpture people? I am anxiously waiting to hear from many schools and haven't heard much from other sculpture candidates. I heard one person got a RISD interview.... anyone else (sculpture) have news on RISD, CCA, VCU, MICA.... ? Let me know!!