First...boy are you correct Scott! You don't read back in the states and you better be prepared to take down a phone number! And if you don't comply, you better have your NASA reporting form already filled out!
Next...it may or may not be in the FARs (or whatever), but I was tought to ALWAYS read back instructions and NEVER to use WILCO (again in the states). The read back tends to keep everyone on the same page-people DO make mistakes from time to time after all.
There are some exceptions to this - to which the response would just be the A/C id, i.e. TWR:"cherokee 08 foxtrot, report downwind rwy XX". REPLY:"cherokee 08 foxtrot".
Fly safe!