I am currently overclocking an ATTiny13A to about 16MHz on INTERNAL resonator, just by setting it to 9.6MHz internal resonator, then writing OSCCAL to 255 (I think only 7 bits are actually used), this then doubles the 9.6MHz internal clock approximately. If you dont need to worry about exact timing then this is great fudge.
I use this for receiving and decoding an 800KBPS data stream and driving a servo from the data and it works a treat as I continually sync on the start/high transition of each bit and sample in the middle, so probably even 1MHz either way would still work
The ATTiny13A does not have pins for a standard xtal, and can only be driven externally by a TTL Clock oscillator, so I chose the method above for cost and simplicity
This overclock can also be done on an ATTiny45/85, so I believe (although you can use a standard xtal with those)