@kg4wsv: I hand-tuned custom lookup tables (stored in flash) for each of 8MHz and 16Mhz. I LOVE the idea of calculating the values, and worked at it from that angle for some time, but I couldn't quite get it to work without fudging the values, so I eventually abandoned the (runtime) calculations. You may have heard the saying "in theory, theory and practice are the same, but in practice they never are". That's what happened here.
So no, it doesn't support 20MHz yet, although now that I know the drill I don't think it would take very long to support it. However, nobody has sent me a 20MHz Arduino to test with, so I guess I won't be able to. :)
@madworm: I'm delighted that the new NSS works for you, although I'm afraid I haven't the faintest idea why. Yours was the only Linux installation I heard of that failed to compile, so I didn't spend much effort after that initial flurry trying to understand the problem. So what's the difference?? Can you tell us?
And thank you for playing with multiple instances.