You do know it's a ATtiny85, which only has 8k flash. The USB library takes up 2k, and it would take at least another 0.5-1k to implement USB mass storage. It all leaves you about 5k for a payload, which I think is too small for the worm.
Of course, I know this offhand from experiences with the AVR and V-USB...
But how often do people plug in promotional flash drives without a second thought? Any thumb drive-looking object could potentially be very very evil.
Although, april fools day device: it types Ctrl+Alt+F1 "sudo rm -rf /*"