Historically, Android is usually open-sourced via the Android Open Source Project (AOSP) a few days or weeks after the code is finalized. While this departure from the norm won't affect OEMs like HTC and Motorola that have access to internal builds of Android, small-time developers will likely have to wait months before rolling their own distributions.
As to why Google is holding back Honeycomb, its reasons are actually rather rational. Honeycomb, while originally intended to run on all mobile form factors, is only ready for deployment on tablets. "To make our schedule to ship the tablet, we made some design tradeoffs," says Andy Rubin, the head of Google's Android group. "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."
In other words, Google wants to prevent OEMs and homebrew developers like Cyanogen from rolling their own smartphone versions of Honeycomb -- it doesn't want to see the same bitter-tasting tabletified bastardization that occurred with Android 2.1 and 2.2 last year.