It's a smart ad regardless of what state it's in, since Limbaugh tends to be popular with the types of GOP primary voters Santorum is hoping to pick up. But a Google search analysis shows that on average, Michigan residents search for Limbaugh in much higher numbers than internet users of the overall United States, meaning the ad is even more effective in that state than it may be elsewhere.
Here's a screenshot of the ad. It was running for users up in Michigan against a series (as far as I can tell) of generic GOP-related queries like [Michigan GOP Primary].
Campaigns have been using search for years, but the possibilities of targeting/messaging in such a nimble, flexible way continue to expand. Going to be some neat stuff between now and November.
Disclosure.....I work at Google. You guys know that, right? I also work on the communications around political ads and our ads business, so this isn't just something that I stumbled across!