Connecting your T-Mobile Blackberry Pearl 8100 to a Windows Vista laptop for use as a modem

Connect the Blackberry Pearl 8100 to your laptop via the USB cable
Start up Blackberry Desktop Manager and make sure it shows connected - I'm using version 4.2.2.14

Click Start, Control Panel then Phone & Modem Options.
Click on the Modems tab, select the Standard Modem then click Properties.
Click on the Advanced tab and enter the following string under Extra Initialization Commands:
+cgdcont=1,"IP","wap.voicestream.com"
Click Start, Connect to, Click on the Setup a connection or network link
Click Set up a dial-up connection and then click Next.
Select the standard modem that you created above and then click Next.
Enter T-Mobile as the ISP Name.
Enter *99# as the Phone number and then click Next.
Keep User name and Password blank. Clear all the other check boxes.
Click Connect.

I got download speeds of 218kbps and 93kbps with a 401ms ping to a San Jose server on speedtest.net

Make sure that you have an unlimited data plan or your bill will be crazy!

If you're looking for instructions for other Blackberry models or carriers, this site has a lot of information:

http://blackberryforums.pinstack.com