Ok, I read the interrupt tutorials/trap thread and couldn't spot where it does say that AVR interrupts are or are not re-entrant definitely. It does say to have critical section or disable then re-enable the interrupt. So, sounds like it's re-entrant.
Just wanted to make that if I do some stuff in my ISR that takes longer than the elapsed timer/counter to expire again, will the ISR be called again. Or not until the current executing ISR exits. If so, I don't have to waste some bytes in writing code to make sure the current ISR finishes executing before it's executed again.