An interesting open* milestone; check the article link below for some speculation about potential reasons and ramifications
"Android 3.0, Honeycomb, was designed from the ground up for devices with larger screen sizes and improves on Android favorites such as widgets, multi-tasking, browsing, notifications and customization," a company spokesman told us. "While we’re excited to offer these new features to Android tablets, we have more work to do before we can deliver them to other device types including phones. Until then, we’ve decided not to release Honeycomb to open source."
Speaking with Bloomberg, Google's Andy Rubin, who oversees the Android project, made similar noises. "To make our schedule to ship the tablet, we made some design tradeoffs," he said. "We didn't want to think about what it would take for the same software to run on phones. It would have required a lot of additional resources and extended our schedule beyond what we thought was reasonable. So we took a shortcut."