I'm having this problem too, with the following notes.
- have been able to upload from the Mac (OS X 10.8.4) previously, including earlier today
- am able to upload from a Windows 7 machine
Currently, if connected to the Windows 7 machine everything looks normal, program runs, etc. I've gone back to just the blink sketch for now. When connected to the Mac, that sketch no longer runs correctly and LED L has a strange blink pattern, on a little long, off, then a set of very fast blinks.
I've been able to switch back and forth between the Windows and Mac machines previously. I updated to Arduino 1.0.5 on the Windows machine just prior to encountering this issue. Mac has version 1.0.5 also.