Unless you are sitting 1ft away from the screen you wont notice that much difference between a 42" 720p and a 42" 1080p plasma at normal 8-10ft sitting distances. Once you start going 50"+ then having a native 1080p starts to make sense,

As true 1080p plasma haven't been around that long your still paying a premium for them so if you thinking of upgrading just for a ps3 then you'll be wasting your time IMO, very few games will be rendering at a true 1920x1080 as the rsx just doesn't have the fill rate, most will be rendering at 960x1080 chances are your current plasma will be more than up to the job.

Another thing is that most 1080p plasmas don't have the same contrast ratio as the 720p ones due to the increased number of pixels, your better of waiting till this improves.