There is some great documentation present in MSDN Online regarding mobile development for SharePoint. One of the great features of SharePoint is that you're able to give mobile device users easier access to SharePoint pages.
If for example you want to make customisations for mobile pages in Windows SharePoint Services there are two approaches according to the MSDN documentation:
1) Create custom mobile pages from scratch and let Windows SharePoint Services redirect users to these pages
2) Override an existing Windows SharePoint Services mobile control template to create a unique mobile Home page by site definition type, or a unique mobile view page according to site definition type and list definition type.
Use the first approach to create an additional custom mobile page from scratch and use it as a Windows SharePoint Services mobile page. Use the latter approach to customize some part of existing Windows SharePoint Services mobile pages through user controls (.ascx files).
Have a look at the following pages:
Mobile Development Overview
Layout and Pagination of Mobile Pages
Some really great walkthroughs and how to's can be found here
Other stuff
Exposing your MOSS WCM to mobile devices
Getting Mobile in Microsoft Office Sharepoint Server 2007
SharePoint on your Phone
MOSS 2007 and mobile views