In 4 states (including Utah), you are required to move to the right if you are blocking traffic. Note that this is exactly what StantonMac said he was doing, so there is no violation of the law there.

Most states (29 of them) you aren't even allowed to go into the left lane if you are going slower than traffic, although many of them have some provision that specifies that you are okay if you are going the maximum speed allowed by law, so this doesn't mean that you are required to speed to be in that lane just because others are.

A few states specify that you keep right except to pass, so you can't camp in the fast lane.

South Dakota has no keep right laws at all- its a free-for-all.

http://www.mit.edu/~jfc/right.html