As far as I can remember, the system-on-chip architecture limits the maximum amount of RAM to 1 GB (I'll try and find an official source, it's probably somewhere in the BCM283x peripherals documentation).

Not to say Broadcom couldn't spin a newer revision of the SoC with a different internal architecture, but for now it seems that >1 GB of RAM would require substantially more effort than just swapping the DRAM module on the board.