> Actually, let me ask you a question.  I play drums and often need to use a
> metronome to set and maintain correct tempo.  This works but has drawbacks
> in live situations where it is nice to let the tempo move around a bit but
> still be at or near the reference tempo.  When using a metronome, once you
> 'drift' off beat you are 'out of sync' with the metronome and its difficult
> to tell if you are still playing at the same rate with a quick glance.

How about this for a device...

Initially, you would punch in the tempo and it would blink the light at
the appropriate rate.

If it detected a "beat" input just before the light was supposed to blink,
it would bump the speed up a little bit.  If it detected a beat just after
the light was supposed to blink, it would bump the speed down a little bit.
By varying the size of its "grab/ignore" zones, you could trade off ability
to adjust more quickly to changes in tempo for ability to ignore notes that
weren't "on the beat".

How does that sound for an approach?  [nb: I'd probably want to add some
phasing logic as well to help the system even more]