12 LED Charlieplexed Snowfall with AtTiny85

  randomSeed(analogRead(0));

Did you disable RESET? If not the pin very likely always reads 1023 because of the internal pullup.

Assuming there is enough Flash available, this is a good way to initialize random...

If you want a truly random seed...
http://code.google.com/p/avr-hardware-random-number-generation/