Tethering with iPhone4 From working perfectly to useless

I recently activated the tethering option on my 2GB data plan with AT&T on my iPhone4. I am using a USB connection to my Windows Vista desktop.
Three days ago, when I first set up the connection, it worked beautifully. I was able to activate the tethering option under Settings -> General -> Network -> Internet Tethering = ON and I was using this with 3G and Cellular Data enabled. About five to ten seconds after I activated this option, Windows picked up the new networking device and I was using the tethered connection right away. The iPhone showed the blue bar connection at the top of the screen for the homepage indicating that the tether connection was active.
Yesterday, when I tried the same steps to activate my tethered connection, I was surprised that it no longer worked. My Windows machine is still able to recognize the iPhone as a device (i.e. I can still see the content on the phone), but it no longer recognizes a network connection. I tried on my Win7 laptop to see if it was something related to my desktop, but it wasnt able to recognize the tethering network connection either. I have an old MBP that I might also hook up to see if that helps recognize the network connection, but I suspect the more likely candidate is the phone itself.
I have spoken with AT&T about the issue and they were unable to provide a working solution. I have performed a hard reset on the phone. Does anyone know of a configuration step I might have missed or alternative methods of testing to try to restore my working tether connection? Id really appreciate any help anyone could offer.
Thanks!
Windows Windows Vista windows