I think I have the same problem, but I do not think it is our fault.
Do you have AT&T cellular service? If so, they have recently changed the tethering settings from using a Cingular server to an AT&T server.
Here is what I know. I had an older version of PC tethering software on 2 PCs. When the Cingular server stopped working, I took a shot that I would have to use AT&T. I changed the settings to AT&T and it still did not work. Then I updated my PC software and selected AT&T again, and now it works. My theory is that the older AT&T settings are from when AT&T was AT&T before it was Cingular and the AT&T settings that work are the new correct ones. The problem with the 770 is that when I changed the non-working Cingular setting to AT&T, it does not work. I suspect new settings for AT&T tethering will fix the problem, but I can not find out how to update the 770 or enter new settings in.
Based on when Cingular settings stopped working for me, and the date on your post, you have the same problem. If you are not using AT&T, then I am off base, but I figured I would post this because at this point, I am having a hard time believing that this has not happened to anyone else. Remember, I had three devices I was doing this with. Two PCs are now working after a software update to Nokia's PC Suite and then selecting AT&T, but the 700 will not work when I chose AT&T.