After wanting to do this for a long time, I finally was able to get my Blackberry 8800 to work as a USB modem for my laptop. This means you can get internet on your laptop/desktop wherever you get ATT/Cingular Service. I got it to work on Vista, but the instructions below are for XP. Vista basically has the exact same setup instructions though. It was confusing at first, but below I wrote out instructions that even a CHILD could follow.

You need the following before we begin:

• Blackberry 8800 (Most likely will work with any ATT/Cingular phone)
• AT&T Wireless Service with unlimited data plan (unless you like paying fees)
• USB connector cable

Okay, you’re ready to begin:

1. If you have not already, download and install the Blackberry Desktop Software.
2. Go to the Windows Device manager. Quickest way: Start > Run > devmgmt.msc
   
3. Click + button next to Modems, double click “Standard Modem”
4. Click the Advanced tab
5. Enter this into the empty box (no leading space): AT+CGDCONT=1,”IP”,”wap.cingular”
   
6. Hit Okay, and close device manager.
7. Go to network connections. Quickest way: Start > Run >
   Paste in the following line and hit enter:
   explorer.exe ::{7007ACC7-3202-11D1-AAD2-00805FC1270E}
8. Click Create a new connection on the left, and follow these steps:
   1. Welcome to … Wizard –> Next
      
   2. [Select] Connect to internet –> Next
   3. [Select] Set up my connection manually –> Next
   4. [Select] Connect using a dial-up modem
   5. Type in a connection name of your choice (example: ATT dialup) –> Next
      
   6. Enter this as the phone number: *99***1# –> Next
   7. Enter user name (ALL CAPS): ISP@CINGULARGPRS.COM
   8. Enter and confirm password (ALL CAPS): CINGULAR1 –> Next/Finish
      
9. Open the desktop manager and plug in your Blackberry via the USB cable. Make sure it detects the Blackberry as connected.
10. Go back to your network connections window (step 7), and double click the new ATT Dialup icon.
11. Your password, username, and dialup number should already be saved. Just click “Dial”

This worked for me on the first try. It will not incur extra charges to your Blackberry plan on AT&T as long as you have the unlimited data plan.