I have 70GB left on my 256GB drive right now. So if I get the OWC/OCZ drive, i'll only start with 200GB left. No good... I'm waiting for the new Macbook Pros to come out, but I'm definitely considering going with an Optibay dual SSD solution as well....
Rumors do say that when Intel issues the Postville refresh in Q4 2010 they will include a 600 GB drive. Hard to believe (I can imagine 300 GB easily), but that is the word in the grapevine. Of course, Q4 might be too long of a wait.