Hey, fanfan! Mega new 24/7 subscriber dittoes!
I don't know this for sure, so don't quote me...but I think it is a reference to the opinion that many of the "mainstream media" types have of Rush, that he just preaches to the choir, and his listeners are just a bunch of mind-numbed robots who get their marching orders from a...well, you fill in the blank.
And since they all agree (anyone who thinks that doesn't listen to the show), they all effectively say "ditto" ("right", or "I agree with that"). So Rush's fans took it as a badge of honor, which either got its cue from, or gave a cue to, Rush. Hence the "Dittocam", the "Dittoheads", etc.
Enjoy the experience! BTW, you should try the podcasting thing. He has much more "on the pod" than just a month's backlog of shows, and now that portable CD players and car stereos are available to play MP3 files, you can literally create a "Limbaugh Library", with months' worth of shows on one CD.
See post # 402, by Shapka, for the Wikipedia explanation.
Cheers.