I couldn't tell from this thread if I2C was supported or not. However, since the ATtiny uses USI (Universal Serial Interface) for I2C and SPI, I didn't think so.
So I dug around and found some code for a USI I2C Master and adapted it for the Arduino IDE.
You can download it here . . .
http://dl.dropbox.com/u/3572198/USI_TWI_Master.zip
Put it in Libraries folder under your Sketchbook.
It relies on the the ATtiny core files from here.
I have included an example in the "examples" folder with an ATtiny85 reading the temperature from a DS1621. It's working fine for me. There is also setup info in the example header.
I'm sure improvements can be made, however you may want to wait a bit. Soon I will post USI I2C Slave code, and it would be nice if the two libs were combined.