I've had a couple of customers come to me with questions around accessing Sharepoint from a Mobile Device.
Firstly there are two different types of access:
1) Mobile Document Access - this is where you use Exchange 2007 to pass through your request for a specific document to Sharepoint and then provide the document to you without having to VPN into the Corporate Network. This requires Exchange 2007 and you have to enable the Sharepoint sites that you wish to access on your Client Access Server (Front End). This solution allows someone outside the corporate network to access a link to a document they have been sent in email without the need for a VPN.
2) Direct Sharepoint Access - It still scares me that so few customers are aware of the Mobile rendered versions of Sharepoint sites in Sharepoint 2007..... if you go to any Sharepoint site and add a /m to the address (eg. sharepoint.contoso.com/m) it will provide a Mobile friendly Sharepoint site. You can't post documents but you can browse in a considerably lower bandwidth environment.
The second issue I've seen is where customers are using protected sites so when you access a Sharepoint site using a PC you get prompted for credentials. When you then try and access from a Windows mobile device you won't get prompted.
This is because Windows Integrated Authentication is enabled. To prompt for credentials you can disable Windows Integrated Authentication.
[Central Admin]->[Application]->[Authentication Provider]->[Default]->Set Off for “Windows Integrated Authentication” and Set On for “Basic Authentication”.