Moving the thread to the iDevices forum.
The main reason to have more RAM at this point would be to keep more things resident - more tabs open in Safari without reloading, more apps open without having to relaunch them, etc. There is some theoretical gain in that a single app could use more RAM for things like say big textures in a game, but that is more about enabling better apps than making an improvement. We just got to 1GB, I don't think anyone is clamoring for more yet.
The downside to having more RAM is that RAM eats power. We all want those 10 hours of battery life and preferably more, and more RAM makes that harder. We also want great graphics, which means high memory bandwidth, which means energy consumption again - and if we want to use those bigger textures I talked about earlier, the memory bandwidth has to rise even more.
In total, more memory will likely have to wait for lower power memory.