Assuming an overwhelming response to this message doesn't hide it somewhere deep in the growing thread, you can now easily refer to a real live genuine Autodesk post for instructions to enable the Revit 2009 UI in Revit 2010.
1) Exit Revit.
2) Create a text file named "debug.ini" in \
3) In the new "debug.ini" file type:
4) Save the file.
5) Restart Revit.
The Revit 2009 UI has really, really, not been tested with Revit 2010, so if you call support and say you're having trouble with old UI, they're not going to be able to help. However, if you're having other issues, you won't be turned away if you have been using the old UI. If support can't reproduce your issue with the new UI, they'll be unable to help, because it remains unsupported with Revit 2010.
As you experiment, we're going to look into scouting out some of the pitfalls you might encounter in using the Revit 2009 UI in Revit 2010 and letting you know what we find.
We remain committed to the new Revit UI and its further development, building on the strengths that many users have found, while addressing the needs of a very large and diverse user community. Rest assured that the entire Revit team has been listening to all the customer feedback, negative and positive, concerning the new user interface and are working on addressing many expressed concerns and suggestions.
It was posted and signed by Anthony Hauck, BIM Design Product Line Manager, Autodesk.
[Edit: The Autodesk Knowledge Base web site article has been updated to reflect this new information as well as providing a DEBUG ini FILE to download. You can read the article too.]
On another note...Phil Read has created a Tee-Shirt in honor of his passion for the classic UI.
You can support it by purchasing it. Watch his blog for further announcements of The 1st Annual Meeting of "The Order of the Drunken Leprechaun" (aka "DLM" or "Drunken Leprechaun Mode").
For an ongoing discussion of quirky limitations stay tuned to this thread at AUGI.