Sources say October 2008. I've heard a lot of people struggling to find a dealer that will take a deposit on one. But I've seen some posts of people making deposits through dealers in Florida. So clearly there are dealers taking deposits. There some interesting posts on one of the Cadillac forums that relate to pre-ordering. Good luck!
I also heard 4th quarter 2008. I'd say October as well. A few people have said they made deposits of $500-$1000 and reserved the car at MSRP. Seems reasonable I guess.