Nokia XpressMusic devices such as the Nokia 5800 or 5530 have a touch button on the top corner to bring up shortcuts to core apps. This functionality has been introduced to the Nokia N900 thanks to MohammadAG from the Maemo forums. The shortcuts are activated by holding the proximity sensor on the Nokia N900.
MohammedAG says this app is more of a proof of concept for now. Head over to the Maemo forum thread for installation and feedback.