I'm waiting for benchmarks to come out. I have a 160GB HD in my MacBook Pro and I always keep more than 100GB free, I just like having a bunch of space. I keep all my files and music on an external hard drive. So, space isn't an issue, but I'd feel most comfortable with a 128GB SSD drive.
Hopefully they will start offering them when I'm reading to purchase my next MacBook Pro early 2009.