How do you do it? There are two steps to this process. First, you'll need to have a Google account to get access to Google Docs. Second, download the Cloud Music app ($2) from the iTunes store. From there, it's easy peasy. Find out what I mean after the break.
After you've set up your Google account and downloaded the app, you can then upload music files into Google Docs for storage. There is a 250MB per file limit, and you can store up to 1GB of files for free (additional storage is available for purchase). Once you have your songs stored in Google Docs (in MP3, MP4, or WAV formats), you'll want to fire up your Cloud Music app, sign in with your Google account, then download your songs to a playlist on your iPhone. What's more, you can upload the album cover art, which will show up when the corresponding songs are playing!
It's a quick and dirty way to get your tunes to your iPhone. As with all workarounds, there seems to be a few hiccups — like, you can't stream them directly from Docs, playback errors, etc. — which you can learn more about on CNET.