SeeqPod has developed a custom version of its MP3 searching technology that autodetects iPhones. The service looks for MP3 files on the Web and lets you play them on your computer or iPhone. From Wired News:
[Y]ou can search for any artist and play their songs within seconds, for free. SeeqPod doesn't transcode the music as it streams, so you hear it in its original form as scraped from MP3 blogs, personal web pages, and anywhere else on the internet that hosts MP3s.
Or, you can use SeeqPod on an iPhone to browse the internet's music pretty much as you would the music on an iPod. Choose a letter, then an artist, and boom — you see a list of songs by that artist that are playable right then and there (thus the tagline: "playable search").