Yes....
I've been working on a solution for months, and STILL haven't figured it out.
But I think it's Android somehow- the way it handles Wi-Fi and 3G at the same time. See, usually when your phone's Wi-Fi is connected to something, it will automatically disconnect the 3G network- I think it's confused- it thinks that Wi-Fi connection to your computer is really to a Wi-Fi network, so it disconnects the 3G network.
What's odd is that sometimes 3G stops working almost immediately, while other times it will work for hours or anywhere in between.
In the meantime, any form of USB tethering works flawlessly.
As for why some people have this issue and others don't, most people don't tether, and those that do tether only do it for a short period of time. Everyone who tethers almost constantly seems to have this issue.