If Sony goes with 512 mb, you can kiss concurrent apps good bye. Sony has never been too good on the software front of things, so I can imagine them side stepping having Facebook / Skype / Twitter notifications running in the back ground.
If I were a betting man, I would say the Vita is a pretty good idea of what Sony is thinking in terms of OS. The Vita OS feedback is probably being used to architect PS4 OS.
They actually suggested during the conference that the Vita OS could be good enough for other devices, hint hint. Would make a lot of sense if you think about it (has all the suspend, small app/large app features, etc.), and improvements they make to it can feed back into the Vita version and vice versa.
I am assuming that when they had 4GB, they couldn't really afford to take more than 512MB for the OS. Now that the PS3 will have 8GB, I would guess that they can afford to raise that to 1GB of OS reserved memory, and although I think many features actually need very little memory (think of what the 360 did with 32MB), it would make concurrent web-browsing significantly more enjoyable, especially when supporting higher resolution displays.