Well, I posted about the power-up RESET delay on AVR Freaks. One thing that irks me a lot is when you post a simple question to a Q&A forum like that and instead of directly answering the question, the experts turn it into a dissertation defense. I do have to admit, though, that in this instance I had to eat a little crow.

In the end, the 5680A's frequency is not stable enough at power-up to directly clock the ATTiny. You have to wait until it achieves a physics lock before it's usable. It's not all bad news, though. The ATTiny does, of course, have an internal 8 MHz oscillator, and - this is the thing I learned - you can actually change clock sources at runtime.

So now the fuses are set for the 8 MHz oscillator at startup, and it runs that way until the !OSC_RDY line from the oscillator goes low. At that point, the clock source is changed to the external oscillator (the 5680), the USART baud rate factors are reset from 8 to 10 MHz, and the code keeps on going.

One nice side effect of this is that you can reprogram the flash or power it up for testing without the oscillator being connected.