Like “high concept,” the term gets thrown around a lot. Used precisely, “wedge issue” refers to an issue that opens a wedge between a given party and loosely committed or uncommitted voters. The bill to create a homeland security department (this is going back a ways) contains a provision that’s unfair to unionized employees. If you’re a Democrat, it’s awfully tough to be against the unions. So the Democrat votes against the bill. He’s been wedged!

If you’re a Republican, it’s awfully tough to be against Rush Limbaugh. If you’re not a Republican, it’s awfully tough to tolerate Rush Limbaugh. Fill in the rest.
Limbaugh could recognize how he’s being used and minimize the damage to what is supposedly his own side. Obviously, he doesn’t want to. He’s selfish. But this is the man so many Republicans have admired for so long.
UPDATE:  Ross Douthat spends a few paragraphs explaining how it might not always be a bad thing that a conservative voices anti-Rush thoughts. To a non-conservative, two points jump out:  1)  Rush picks a lot of fights with other conservatives, and 2)  he doesn’t try to start ideas, he tries to stop them. He’s the orthodoxy police. Douthat is reduced to sounding like a Soviet reformer: oh, for “some space in which to experiment a little.” 
From reading the Douthat post and about Frum’s dust-up, it sounds like Rush slaps but no one is allowed to slap back. He can say you’re wrong; if you say he’s wrong, you’re a traitor. How did a movement ever wind up like this? Seeing the conservatives put up with Rush is like watching a grown man pick his nose. It’s disgusting and surprising and you wonder how he can think it’s normal.