You can build to order the 17 inch with a 256GB SSD.
Which obviously means they are available.
Also, SSD's will be released in up to 1TB models in the near(?) future. One article here on Engadget. The Register Hardware article says the pureSilicon Nitro will "also be available in ... 256 and 512GB capacities".
EDIT: forgot to say, no pricing on the above drives as yet. 256GB SSD's can be found on Newegg here.
The way prices have gone down recently, I would say to expect a 512GB SSD by the end of the year with 256GB ones being fairly affordable and the 512 ones affordable by next summer, unless of course you find 750$ or so affordable 😛