The playstation store is finally getting awesome.

[Shadowdust]

Aside from legitimate ownership, are there any differences between the downloads and the original discs?

Nope, just the convenience of not popping in a disc as well as the ability to transfer to your PSP.

I know Sony have said in the past that they will not release PS2 games on the PSN for the PS3 so I don't expect them to do it. Especially since the GOW collection shows they can repackage games in HD and make more money off of them, not to mention they're still supporting the PS2. Seeing as how the PS2 still sells well and makes a fair bit of profit on everything, I'm not so sure they'll want to undermine that in the near future.

On the other hand, if Sony wants to encourage PS3 sales, at some point it would make more sense to emulate PS2 games on a PS3 and discontinue the PS2 console. As you said though, for the time being, the PS2 does still sell well so it won't be in the near future if it happens at all.