If you're not using a tethered plan (i.e., the MediaMAX 200 plan) then isp wont work. You need to use wap.cingular for non-tethered plans.

It works perfectly for me with my Nokia 6682, N93i and Samsung SCH-A717. Just use the N800's default setup for the Cingular network, but change both instances of isp to wap. Should work fine.