In reply to Si dH:
We did a bouldering trip to Utah about 10 years ago. We didn't actually get that much from the bouldering because the skiing, boarding, crack climbing and mountain biking were so good. Why do merely absolutely amazing bouldering when you can do the best cracks, riding, mtb tracks in the world period.
But to answer your question..
Little Cottonwood Canyon. Big grantite blocks on the way to Altah/Snowbird. Very good.
Joe's Canyon, halfway between SLC and Moab. Huge sandstone area in the middle of nowhere. Perishingly cold at night. Good climbing.
Big Bend at Moab is just two solitary boulders. Great location, but seems perverse to boulder when there's so many other BesINTheWorlds nearby.
enjoy.