I know this is probably a dumb question... Can you put the Arduino bootloader on the ATMega328 and plug it into an Arduino board?
Please, someone, tell me if I've got any of this wrong.
First point: The new chip is pin-compatible, so, hardware-wise, upgrading a classic Arduino with a socketed processor is about as easy an upgrade as possible.
Second point: Unless you change the definitions in the development environment, and (at least) the landmarks in the bootloader, then the software can't know that there's more space available, and consequently, even if it would work, it wouldn't work any better.
Third point: It seems that the next release of the Development Environment, release 13, will support the new chip, with an appropriately tweaked bootloader. At that point, those of us that can change a socketed chip (if not much else) will be able to download the new software and swap-up the main chip to enjoy about twice the space for our programs to play in. But for now, its a little more complex than that.