For those Crackberry (err, Blackberry, Palm and other mobile devices), providers can use Funambol to provide PIM and push email. There are other open source Blackberry tools you can use / offer. (Look at Open X-change too.) Now Mozilla is adding mobile content.
Mozilla Corp. developers are experimenting with a service that lets users store online content on a remote server and access that information on cell phones. [source]
If you service business customers, this is a natural add-on to email and hosting.
So is offering unlocked cell phones, PDA's, EVDO cards and working with an authorized cellular agent.[Update: there are other add-ons that you can offer from the Web 2.0 or Voice 3.0 realm, like iLane, to check email hands free].