With 20 million listening you have to be careful what you do, and or say, he is.
I think you've hit on one of Rush's chief considerations about publicly backing a candidate during the primary, but once Palin declares, it would be dishonest of him in the extreme, to not do something to indicate his support for her to his audience.
I know that this is a delicate matter for radio talk show hosts. Rush isn't the only one who has to carefully watch how he treads during a presidential primary. We're only talking about him, because he's the most influential and well-known conservative talker, but all of them will have to face the same question.
My guess is Rush is paranoid about a possible scenario where Palin declares she is running and then a Romney or Newt wins the nomination.
At that point he does not want his audience to stay home or lose enthusiasm as the stakes are just too high.
Keeping that in mind,and it's a possibility however remote,its hard to see him coming out,but you never know.