Before I invested in a decent MP3/CD head unit for the car, I used to use my phone
Its a K750i, 2 years old at least. Has a 2 gig memory stick in it, which is more than enough for listening to music in the car or bus.
I bought the adaptor which lets you plug in your own 3.5mm jack and connected that to one of those tape units (budget I know, but it was a VN commodore before the GTR and I wasn't about to spend money on it ;P)
Anyhow, you can now get more recent sony ericsson walkman phones with the 3.5mm jack out already, so you can use it as a walkman, and then get a m-m 3.5mm cable to go from the phone to an aux in of a head unit... (or the tape thingy)
I highly recommend it. The MP3's are stored on the card of your chosing and detect as a simple usb drive when the phone is plugged in.
-D