There has been some confusion about whether you can compile iPhone SDK 2.2.1 applications once you've installed the 3.0 SDK. There were conflicting reports swirling around the Twitterverse for the first twenty-four hours after release, but we seem to have a definitive answer now from some nice folks at Apple.
The answer is: Yes, you can compile 2.2.1 (and earlier) applications, but DO NOT SUBMIT APPLICATIONS TO THE APP STORE THAT HAVE BEEN COMPILED USING THE VERSION OF XCODE THAT GETS INSTALLED WITH THE 3.0 SDK.
This is really only a minor annoyance, since can have multiple copies of Xcode installed on your machine. The "Read Me" on the disk image tells you how to install to a non-standard location. So, if you have applications on the App Store, or have ones you plan to put on the App Store before the GM release of 3.0 this summer, make sure you keep your old developer tools and install the 3.0 SDK tools to a new location.