One of the most interesting things that SharePoint 2007 brought out was mobile support being built in.  Each list has it's on default mobile page that you can use to access information, but there's a good bit more functionality there to handle redirecting mobile browsers to the right page when they get to your SharePoint implementation.  I've been going through some of the updated MSDN documentation on SharePoint (See previous SharePoint post) and there's a nice overview for development for mobile browsers at (http://msdn2.microsoft.com/en-us/library/ms464268.aspx).  Additionally there's a good location to answer that ever present question "What can I actually use" when thinking about mobile browsers - the list of Mobile Controls that you can use is at (http://msdn2.microsoft.com/en-us/library/microsoft.sharepoint.mobilecontrols.aspx)