Possibly later this year, in the fall. No one really knows for sure.
But remember that SSD prices are still really high. $1000 to upgrade to a 64 GB SSD right now, and I think I read that 128 GB SSDs sell for over $3000... I would imagine that Apple will not offer a 128 GB SSD until the price gets down to the $1000 level, because I can't think of anyone (myself included) who would drop $5000 on a notebook.