As long as your phone supports it, you'll be able to add up to 12GB of storage with Sandisk's upcoming high-capacity MicroSD card, which will turn cellphones into even more viable music players (assuming their controls don't suck).
The company is trying to raise awareness about the idea of using a cellphone to play back music with its WakeUpYourPhone site.
There's no price or availability at this point (a Sandisk spokesperson said this is a "technology announcement"), but we expect the card to appear sometime this year, this being CES 2008.