icingonthecupcake is just okay, I have tried the cupcakes at El Charro in old historic roseville though and loved them. El Charro makes 4 kinds, Kahlua, Carrot, Red Velvet, and Double Chocolate. They are big too not like the ones at icing on the cupcake, and sell for 2.50 each.
Sactown magazine's Feb/March issue just had an article profiling the 10 best places to get cupcakes in Sacramento.
So it sounds like there are three cupcake only bakeries and then several cafes and coffeehouses that also carry cupcakes.
I actually picked up the magazione in a so cal Barnes & Nobel and haven't been to Sacramento since these bakeries opened so I have no personal reviews to add.