Hi MikeD,
Floating point is wasteful processing time (and memory) and should be avoided if possible in code that is time critical. Also, the code posted above uses a prescaler of 256 so with a 16mhz clock it can have an error of up to 127 microseconds.
Here is a thread that has a discussion on precise timing on the Arduino: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1226014847