Sense 2.0 can not rotate to landscape mode. It was supposed to be written into the Gingerbread update according to HTC but was not included for some reason. The only way to get home screens to rotate into landscape is to root and use either a vanilla rom or a rom running Sense 3.0. It is a limitation of Sense and not of Android proper. You can try bugging HTC to include it in a future update but since the hotfix they promised after the Gingerbread update has yet to appear to correct the programming errors that were in the update (see threads about widgets and programs disappearing) I wouldn't hold my breath. Your best bet is to root and go to a different ROM.