I got my new Treo a few days ago, and with the exception of a few minor quirks (Why on Earth did they not add a backlight to the keypad buttons??), so far, I find it to be a great little PDA/Phone combination from Handspring. You can not only do IMs, but access the real (HTML) WWW and e-Mail, among a number of other things.
But, the Treo doesn't have a springboard expansion, or any other kind of expansion, for that matter. Besides, even if the Treo could handle MP3 playback, you'd be stuck with a mono speaker not meant for quality MP3 playback. So, I'd wait and see if the Hiptop device lives up to its promise, or just buy a seperate MP3 player. Although the iPod is the coolest and best one out there, you could buy one of the cheaper ones if you don't require all that iPod coolness, features, and so forth.