For $25 on top of any Alltel voice plan you can tether any 1x/EVDO phone to a computer (and get rid of your AT&T DSL service). Tethering on a non-smart phone requires Quick Link Mobile software to be installed on your computer though and the phone connected via a USB cable, so on further reflection this is probably not what you need.

Otherwise, any phone is Axcess Web capable. There is no cost, except that it takes away your "peak time" minutes (M-F, 6AM-9PM)of your voice plan. On nights and weekends you cannot incur "peak time" minutes so data access is free. Alltel offers a package called "Axcess Minutes" for $10/month that allows your data access on your phone to not count against your "peak time" minutes, thus you would have unlimited data for $10/month on top of any voice plan.

Someone else with experience might have to verify my second paragraph to see if this is the way Alltel or their provider offers the service that will allow a Bluetooth connection to share the phone's connection, or if the method in the first paragraph is needed to share your connection with your N800.