As described in this post on an Arduino forum, I’ve suggested some small changes to the delay() and delayMicroseconds() code in the Arduino/Wiring libraries:
(rest of delayMicroseconds unchanged)
Note that both changes only add new code, no lines have been removed or changed (the comments need to be adjusted, though).
The above changes only affect microsecond delays > 500 us and millisecond delays < 50 ms. In that range, a loop is used which should be accurate in the couple-of-microsecond scale and which does not lock out interrupts.
It should lead to a more accurate millis() value since interrupts can be serviced more often, and to a far more accurate delay for low-millisecond values. I hope something like this makes it into a future release of the Arduino IDE – timing capabilities are still a bit limited on the Arduino, IMO.