I'm using the Arduino TinyWireS library, which is a pretty thin wrapper on top of Donald Blake's usi_twi implementation based on the AVR312 app note.
Observed behavior: If I set my i2c bus for 5KHz, I can communicate with the attiny84. If I set it for 50KHz, I can't. The controller in this case is a bus pirate, which has about 10k pullups, and the SCL curves do look a little more rounded than I would like, so I tried adding 4.7k pullups, and... no real change, I guess. My assumption is that an 8MHz CPU should be able to handle i2c at 100kHz or 400kHz, but certainly without any difficulty at 50kHz, but at 50kHz, it just doesn't even get noticed. I've poked around and read the source and the app notes, and found nothing at all specifying any particular expectations as to the speed that should work with the USI hardware.