I've seen Jason do his stuff on Letterman and was interested to see John Carney.
Frankly, given the circumstances, I thought John did a good job. Understand that when close-up magicians were on the Carson show Johnny had a table in front of the curtain and he and his guests left the desk and sofa and went over and sat in the magician's performing area. John was both a magician and a fan of good magic and no performer ever had to worry about him butting into the act or making untoward comments. You could shine on the The Tonight Show with Johnny Carson
Working Letterman you dont have that surety. At heart he is a horses behind and loves scoring points off people who come out. Carney came out and had to perform sort of side saddle turning to the side to work off of Lettermans desk. Letterman had to clear away stuff before John could perform which made it look like the performance was an afterthought and that no one had prepared for it which I see as a form of disrespect to the performer. How difficult would it have been to bring out a table, a table cloth, and a few chairs, inviting Miss USA to join Dave and John at the table?
John also had to bite his tongue and tread lightly when Letterman started making comments as he moved through his routine. Letterman well knows the need for rhythm and pace in delivery and seemed happy to interrupt John as he tried to establish a smooth pace.
Cher nails Letterman with the truth at about 3:30 the first time she was on the show. See: http://www.youtube.com/watch?v=LBgDHhmSrAo