Me too
17" 2011 with the same problem, only no tethering involved. Just crashing. Happens very often, several times a day sometimes. Seems to be happening more when downloading large files via firefox. Crashed on me 5 times in a row with a 67Mb aif file. Finally switched to Safari and it downloaded without crashing. For me, maybe Firefox is the culprit. Going to start paying more attention to whether FF is open when it happens.
This is by far my most loved and most hated computer ever! Have 4 other macs in the house and I've seen that screen maybe 6 times ever before this computer. Now, anywhere from once a week to 6 times a day.