I've got some good news today: the Android 1.5 SDK, release 1 is ready! Grab it from the download page.
For an overview of the new Android 1.5 features, see the 1.5 release notes page in our developer site.
I am also happy to let you know that our partners at HTC have made available new system images to upgrade your Android Dev Phone 1 (ADP1) to Android 1.5. This new version (which is only available for the ADP1) is based on the Cupcake branch from the Android Open Source Project and corresponds to the system image of the Android 1.5 SDK, release 1. If you have questions about the process of updating your device, you can ask the mailing list that we've set up.
I'd also like to note that Android developer phones like the ADP1 are intended for application development, rather than daily use. Additionally, they are operator-neutral and country-neutral, so they may not include certain features found on end-user devices.