If you're primarily going to be watching SD content on the TV and you now say you're getting a seperate 24" for pc use, I really don't see why you need 1080P. I know you say you want future proofing but when are you going to switch over to primarily HD content? If it's not going to be within the next couple years, you should just buy a 720P plasma, pocket the money you save and buy a 1080P set when you can actually take advantage of it. We have both 720P and 1080P sets in our house and Blu-rays and HD-DVDs look nearly as good on 720P as they do on 1080P (Distance of about 5ft away) if you purchase a decent tv.
1920x1080 helps with using PC as a source, display native and no scaling.
It's true that 720P is way too low of a res to be used for daily pc use however he has said that he is purchasing a 24" for primary pc use and that he will only be connecting his laptop for occasional gaming and films. Unless he's running something with a 9600m or higher, I assume he won't be gaming at 1920x1080 so he doesn't need it for that. Films, unles he's watching 1080P rips, 720P HDTV and SD content from his pc will look better on a 720P set. Also unless his laptop is really old, you can just select the tv's resolution in the graphics settings and display the laptops video signal natively with no scaling.
The "720P" plasmas I've seen are 1024x768 which most laptops can output natively. The "720P" lcds I've seen are 1365x768 which some older laptops won't output natively, most newer laptops can output that res natively though.
i think im gonna have to get round my uncles house, he has a samsung lcd 46 inch, see what i like best, [generally] i think both the 50 plasma and his lcd are good tvs, at least i can decide plasma or lcd! even thats too hard atm