It looks like Don Kinzer is off to a good start with it (in hptics):
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1226257074/1#1
though with the divisions it is a bit more computing than I'm proposing, which at it's core could be expressed as:
((m << 8) + t) <<2;
I wanted to keep it light so you can get a new microseconds timestamp from an interrupt without much concern, so it overflows every 128 seconds internally instead of computing the seconds portion as well, but just adding timer0_seconds * 1000000 will get you there.
But I figure being able to track any number of signals at low microsecond accuracy with a period of up to just over 2 minutes is pretty useful.
Like so much of microprocessors, always a compromise 