When doing mobile websites, I tend to build a whole new area, instead of using the PDA/Smartphone support. This is because i don't want all content from the original website to be shown on the mobile site.
In this process, the ability of setting a canonical link from the original page to the mobile version, could be very nice. It could also be used to make a javascript redirect from the original page, to the mobile version, in case of a visit from a mobile device.
Also, I don't know if it's already possible, but a feature for creating mobile XML sitemaps could be very useful.
