Samsung i500 Online User's Manual page 198

Can I sign out of data services?
You can sign out without turning off your phone, however you will not be
able to browse the Web or use other PCS Vision Services. While signed
out, you can still place or receive phone calls, check voicemail, and use
other voice services. You may sign in again at any time. To sign out, tap
.
Section 3: Using PCS Service Features
3B: PCS Vision 195