Michael Harrison has developed two utilities for the Tablet PC and UMPC user: Rotate Screen and ClearType Rotator. I have not tried these out myself, but they look quite useful. Until Michael mentioned to me about rotation problems with ClearType, I didn't know the problem existed. Interesting.

Here are the links to the utilities and a description from Michael on the problems they are designed to solve:

1) RotateScreen

RotateScreen is a very small console application that allows the user to rotate the screen to a specific orientation.  It's designed to be called from another application such as a tablet menu.

Download: http://www.dragonseye.com/RotateScreen100.zip

2) ClearType Rotator

I finally got tired of resetting ClearType whenever I rotated my tablet screen and started work on ClearType Rotator.
It can capture your font smoothing settings for each screen orientation and automatically restore those settings when you rotate your screen.  It can also rotate the screen to a user-specified angle.

Download: http://www.dragonseye.com/CleartypeRotator002.zip

Thanks for the correct link, Robphy.