Couple things... Sprint does indeed still offer the software. Go to
http://xrl.in/21ic and select your phone, if you have the centro its there on the first page after you click the smart phone link.
Download the Connection manager for phone as modem.
If you want to be more stealthy about it, you can use PDANet or USBModem. I've been using USBModem for years and I love it. Even though all my smart phones are flagged for phone as modem on my plan, I use the USBModem software anyway. Works like a champ.