I've actually looked into replacing the 256MB Toshiba DDR RAM chip, but it looks like it's combined with the OMAP processor in one big IC.
Well, that's the direction that phones are going. I'm hoping that one day in the near future, when I need to use a "computer", I'll go to a setup that has a monitor, keyboard, and a mouse or digitizer, along with a standard connector. I'd plug my phone into that standard connector and it would allow me to work on a larger, computer-sized scale using my phone. Who wouldn't love to have a phone powerful enough to do pretty much everything we do with computers today besides gaming and intensive computation? Better yet, you could take your computer with you to any location and as long as there's a desktop setup like the one I described, your settings, programs, etc would still be intact and you could continue your work on the go.
As much as I love my computer (I'm an avid PC gamer), I would also love to replace it with my phone for everyday tasks and save it for the fun stuff if phones were powerful enough and the connectors were around to scale it up to computer-sized user input (full-sized keyboard and monitor, etc).