In my work as consultant I got many questions about the usability of mobile devices. Especially in Europe you will see lately the promotion of device that can be constant online with access to (high) speed internet.
There are two solutions available for the 3.0 version of MS CRM a 'browser' and a 'Windows Mobile' based solution. For devices (smartphones, Palm, Blackberry) that support html 4.0 you could use Offline Mobile Express client (see CRM_MobileExpress. Mobile devices running on 'Windows Mobile' could use Offline Mobile Sales Client (see CRM 3.0 Mobile Datasheet).
From an user perspective I would recommend presenting not to much data, because the screen is very small. It works better with devices where you can flip your screen from horizontal to vertical so more space is available. For example the HTC P4350 Pocket PC Phone
Update - March 19th 2007
Another mobile solution for MS CRM 3.0 comes form CWR Mobility, reviewed last year by Ronald Lemmen