Is microsecond precision available in an iRule?

microsecond resolution isn't currently possible, but millisecond resolution should work fine for calculating deltas in time:

set start_time [clock clicks -milliseconds]
 do something else
log local0. "delta: [expr {[clock clicks -milliseconds] - $start_time}]"

http://www.tcl.tk/man/tcl8.4/TclCmd/clock.htmM5

clock clicks ?-milliseconds? ... This value should only be used for the relative measurement of elapsed time.

For the current time, I think the best you can get is second resolution.

Aaron