just go to your display settings and uncheck all the resolutions except 1080P and it'll run it in that resolution... same goes for any other game. (i figured this out with batman: AA, and since then all 720p games i been playing at 1080P)
It's probably not a great idea to force games to play in 1080p if they were designed to be played in 720p (like Uncharted 2). The only thing that happens when you force it to play in 1080p is that the Playstation forcibly upconverts the game resolution to 1080p, but most of the time you can visibly see a loss of texture quality by doing so. Since your television automatically upconverts pretty much anything to 1080p (if that is your television's native resolution) and the game has Anti-Aliasing, the game will probably run and look better in it's native resolution at 720p.
If you own an LCD (or really, anything other than a CRT) then whatever is sent to the TV is scaled to the resolution of the set. So, if you have a "True HD" LCD that has a resolution of 1920x1080 then *everything* that is sent to the TV is scaled to 1920x1080.
This is why up-scalers are overrated ... because your TV is already doing it for you. And if you have a good TV with a good scaler, it is likely doing it better than any DVD player could (including the PS3).
So, test the "1080p only" trick and see if it looks better. If it does, it means your TV has a bad scaler. Typically, you'll want the PS3 to output what the game wants to output (e.g., for Uncharted 2, that'd be 720p).