The Backwards compatibility isn't the problem, as if Sony did release PS2 games on the PSN store, they would release a firmware update that emulated them. They could add a few different games at a time, and update the firmware as need be for each new batch. Of course, the PS2 emulation would be built into the PS3 firmware, so any games that appear on the store, should be playable if you have the disc copy without the need of re-purchasing the game.
It comes down to Sony whether they see the value in doing so or not. I imagine they will get round to it eventually, but they would need to make a much better effort than what they have with the PSone titles. Game size wouldn't be too much of a problem, most games on PS2 averaged around 2gb in size if I remember correctly, and I'd say if your downloading more than one PS2 game per month, then your probably able to pay for a larger download cap.
Would like to see it happen, but I would also like to see more PSone games turn up on the store every week too. It has been rumored for a while now, E3 perhaps? I'm not holding my breath, but it would be a nice surprise.
