Currently there is no way to lock the N900's UI to to horizontal mode, which gets bothersome in cases. Applications that support portrait mode often have a setting, but that's not true for all.
There should be a hildon-status-menu plugin that disbles the accelerometer for the UI when activated, much like the iOS 4 one: http://www.tech-recipes.com/rx/6125/iphone-ios4-lock-orientation-but-where-is-the-horizontal-lock/
I currently have a shell script that edits /etc/mce/mce.ini, removes the accelerometer entry on the Modules to load list then kills mce. It works, sometimes, except when mce doesn't return and I have to restart the phone. I'd love a less hack-ish solution to that.
