Some clients insist on having mobile broadband cards in their laptops which is pretty cool.  But why pay for the extra data plan when so many of these same users already have a smartphone with data.  Just configure the device and laptop and you can access the internet wherever there is a cell phone signal.   I have gotten BlackBerry and Windows Mobile devices up and running in this way (on AT&T and T-Mobile, other carriers might block this and try to charge extra for it).

Here is a guide to setting up Blackberry devices as “modems”
Windows Mobile setup is pretty simple:

I haven’t setup Treo’s for this..