MusicIsBliss:
Firstly, I have downloaded arduino-tiny-0150-0020.zip from Google Code Archive - Long-term storage for Google Code Project Hosting., and placed it in "arduino-1.0.1\hardware".
For everyone else reading this thread...
The 0150 in the download name indicates it is for Arduino IDE 1.5.x. You need a download that starts with arduino-tiny-0100- or just click Download the latest version for Arduino 1.0 from the home page...
http://code.google.com/p/arduino-tiny/