Yeah, a handheld scanner is what you want. The Radio Shack radios are nearly always older model name-brand radios that are rebadged for RS. They are usually cheaper than buying a name brand, plus if you keep your eyes open and wait they have excellent specials from time to time.
The Icoms I mentioned have a lot of nice features, and I mentioned them because I've seen them specifically marketed to racing fans. Really more for track racing, since the radios have gobs of memory channels that you can pre-program to the frequencies that each team will be using. But they also have wide band coverage from like 4khz up to 1ghz (except cell frequencies of course,) which makes them useful to play around with to listen to weather reports, police/fire/ambulance, boring ham and business communications, etc. But most scanners have that these days.
Basically memory channels are the most important feature you want, enough to hold all the frequencies that the rally will be using (usually 4-6) and local emergency frequencies as well (look them up on the internet before you go, you can find web sites like the Long Island Scanning Resources
http://www.fordyce.org/scanning/ which covers most of the Northeast and DC regions). Sometimes the rally freqs are published at HQ, if not then you may be able to get a list with press credentials. Sometimes the radio workers have a private secret unpublished frequency (e.g., STPR) that only they know about and is not announced until the radio worker meeting. You want to make sure that you can put the scanner into a mode to scan only those memory channels. I know that the Icoms let you have multiple memory banks, so you could for instance program the rally freqs into one bank and the emergency freqs into another, and limit your scanning to just the rally ones unless there is an emergency, a nice feature to have (do you really care about the domestic dispute across town?

)
Also battery life is an issue, make sure it lasts long enough if you have the radio on constantly or you can charge it in your car.
Any handheld will have an adequate antenna to receive. They usually have one of those short rubber antennas which you can usually replace it with a longer and more efficient telescoping antenna, as long as you know what kind of connecter it is using (the RS guys will know, there's always at least one or two hardcore guys at every RS that know more than you could ever possibly want to know.)
edit: PS I forgot the most important feature, you want to make sure the radio actually picks up the rally freqs. They are almost always on the 2-meter band, aka 140-144 MHz. Being able to pick up the 440 MHz band is a plus but I've never seen it used in rally. Don't know off the top of my head what bands the emergency frequencies are on, but you can check the website I listed for examples.
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JP Rowland jeremyrowland -at- mac.com
Co-driver, Wazoo Racing Subaru Impreza WRX, Production GT Class
http://homepage.mac.com/jeremyrowland