The latest HTC UI called Sense was ported recently to older Windows Mobile phones. While it is a really nice UI it doesn't work in Landscape mode. This limitation is now gone thanks to talented folks at XDA-Developers.
A CAB file was recently released to make HTC Sense work in Landscape mode, you can find the files and the installation instructions at http://forum.xda-developers.com/showthread.php?t=588476.
The install is quite simple:
1. Install Mortscript, you can find it here: http://tp2.blownfuze.org/files/MortScript-4.2-PPC.cab
2. Disable HTC Sense (Manila) prior to installing the CAB.
Go to Start menu > Settings > Home > Items and uncheck HTC Sense.
Install CAB file found here: http://www.mediafire.com/?tintnoyzzmy or here (if the other one doesn't work): http://tp2.blownfuze.org/Rotastrains_landscape_2.5.cab
It will take a while to install, be patient.
3. Install a registry tweak found here: http://tp2.blownfuze.org/rotastrain_fail_safe.cab.
You will find some screenshots at http://forum.xda-developers.com/showthread.php?t=588476.