Checkpoint have recently launched an SSL VPN solution for Windows Mobile devices that provides some very interesting capabilities.

Firstly it supports seamless roaming between networks (3G, WiFi etc..) whilst maintaining session and caching credentials. 

It means that you can have a mobile worker solution working irrespective of whether they are in the lift or underground.  Connectivity will resume once it is available and you don't have to reauthenticate/resume the session - all this is handled automatically!

You can read more details on the Checkpoint announcement HERE