The Biles, do you have the PAM plan from sprint or just a normal phone plan?
Also with centro/treo from sprint and DUN in OS2008 Diablo you have to make sure username and password fields are blank. I had to go into my connection manager and delete <Username here> thing that it puts in by default. If you don't have a phone as modem plan and are using USB modem make sure you diaable default dun VIA usbmodem and set usb modem to bluetooth mode, then the N810 will connect as normal using sprint as DUN on the N810 with user and pass blank