Not really hypocrisy.. more of a double standard. On the flipside, allowing third party apps to run in the background would require some new UI implementation. Right now iPod and Phone are the only apps able to run in the background, but they basically run all the time. For music apps it could be as simple as just checking if music is playing when you press the home button. If it is, then it stays open, if it isn't then it quits out of the app. However, for other applications.. it could get a little bit tricky.
I bet that they will come around next year with 4.0, but I bet it will be a feature that only the 3GS and the 4G iPhone will support.