I used to use an iRiver H10 20gb with Rockbox open source software (as it added some features that the base firmware didn't have). However, the battery was ALWAYS crap as it was a display model which was on all the time, but I got it for 60% off.
This year, I got a BlackBerry Storm and I have been very happy with it. 16gb memory card, I can run Slacker and Pandora radio and I also purchased PodTrapper to pull podcasts over the cell network. I might have considered getting an iPhone if it weren't for the fact that I would have to use AT&T and the coverage sucks for them in my area.
If you ever decide to go the combined phone/media player, I highly recommend checking the phone AND network reviews for the one you want. If you want to get a new dedicated media player, you might be able to get an older model and enhance it with RockBox.
http://www.rockbox.org/