Looks like you've already got your answer RM, but here's a little more info.
That Mayors interpretation actually gives recent clarification to the FAA's position that time spent on-call is considered duty time specifically for the purposes of part 135. Long before that, though, there were many interpretations that specified that time spent on-call could not be considered rest time.
No matter which part one operates under, a legally sufficient rest period must be: a continous period of time, assigned in advance, and free from work or the present responsibility for work should the occastion arise. Present responsibility = Duty.
It's really quite clearly spelled out in 135.263(b), though many people would like to ignore it, thus all of the Chief Counsel's opinions.
§ 135.263 Flight time limitations and rest requirements: All certificate holders.
(b) No certificate holder may assign any flight crewmember to any duty with the certificate holder during any required rest period.
Some charter operators even pushed the issue as far as the United States Court of Appeals at one point, and the Court shot them down.
Here are a couple of links to threads which have discussed this at length and include links to various legally binding documents:
http://www.airlinepilotforums.com/pa...-not-rest.html
http://www.airlinepilotforums.com/pa...l-vs-duty.html
Good luck!