Something the iPhone is missing is internet music streaming. A web application called SeeqPod addresses lets you listen to music files publicly available on the internet. Point your iPhone to SeeqPod.com/iPhone and start clickin'.
You can either browse by artist or "search and discover" on the home page. Browsing has a really pretty, iPhone-esque inferface.
The discover function is really neat. You type in an artist and it'll bring up ones that are similar to who you typed in. A search for Franz Ferdinand brought up Joss Stone, The Killers, and Coldplay, for example.
A few problems: browsing can be a little wonky. Since the music is just indexed from the web, some of the songs aren't named, just file names like "Audio Track 06" by Elton John is actually a live version of Rocketman. Going between tracks and artists can be a little slow, but it's understandable as it's loading webpages as you do it. Another problem is the inability to download the song to the phone, but if it were possible, I'm sure you'd run into a billion copyright issues.
I tried this over the Wifi at my house and over EDGE. Wifi was fine and fast, just as expected, but over EDGE, that was a different story. The MP3 files take a good long time to download, so I wouldn't really recommend using this site via EDGE unless you're horribly, terribly, impossibly bored. It's a little bit faster than dial-up. Frustrating, but doable, but "rather not"-able.
SeeqPod is definitely going in my "Apps" bookmark folder in mobile Safari. Seems like it'd be a neat way for your friends to recommend new musicgo to the URL, click "share" and email away.