I am also having this problem on the Arduino Robot:
The problem presented itself after uploading a number of example sketches. The last sketch that was uploaded, works fine but no new sketches can be uploaded. The identical problem occurred on two Arduino Robots. The only difference was that different sketches were uploaded.
I have tried all the solutions I could find on this forum, such as pressing the reset button at different time during the upload. Since everything worked so well initially, I tend to think that somehow the boot-loader may have been corrupted.
I am about to purchase a In Circuit Programmer from Digi-Key to reload the boot-loader.
I wanted to know if anyone had any if anyone found that reloading the boot-loader solved the problem or if there is abetter solution. I have never experienced this problem on the UNO board. I wish that I could be more help but I am also looking for help with this. I wish that this problem could be brought to the attention of the Arduino Development Gurus because it is serious and in my opinion, beyond the ability of causal users to solve. It seems to be an Arduino firmware issue.