Just wanted to add my voice to the chorus on this issue. I'm using a Mega 2560 R2 with Arduino 1.0.1 running on Mac OSX 10.6.8. Changing "stk500v2" to "wiring" has made no discernible difference.
As a few users on this thread and others have found, it has been a sudden behaviour change where I can't think of anything I've changed...
EDIT: to add a couple more details:
- The sketch that is currently loaded still functions perfectly.
- (non-)behaviour is seemingly identical in Arduino IDE 0022, 0023, and 1.01.