Ever since Google bought Simplify Media and killed it for the iPhone, there has been a need for a reliable way to get your DRM-free music library to your iPhone. Audiogalaxy has done that, and quite a bit more. Audiogalaxy basically streams your music collection, using a helper app, to another computer, an iPad, iPhone, or an Android phone.
To get started, you sign up for free, and a Web-based client will scan your hard drive for your music collection. If you have a lot of music, it can take a lot of time, so be patient. When that's done, you can grab the free iPhone or iPad client, sign in, and your music is with you. The app works fine over 3G or Wi-Fi; audio quality is good, but it's not CD quality. I tried the app several times, and it worked well on a three-bar 3G connection. The app also supports background play in iOS 4.