The cup warmer should be called the cup burner since they can get so hot up there. I've noticed some strange happenings lately with preheated cups, they get so hot that it changes the flavor of the coffee (depending on how hot you let them get).
I did a test with a warm cup and a scorching (7th level of Hell) hot cup side by side. I pulled a shot with the double spout simultaneously into each cup and the espresso in the super duper hot one was bitter and quite normal in the other one. Another phenomenon I noticed is that the Illy Cups get hotter than my other brand of cups. I mean really HOT, not sure if it's a different clay they use for the porcelain or what...but you can BURN yourself if you pick up an Illy off the cup warmer.
Time to bust out the Fluke and take some cup warming measurements....how geeky is that?!