I'll add my speculation as well, and that is that while dirty bombs probably can be delivered via a missile system, nobody would probably bother. Like Kap said, dirty bombs are low-tech "suitcase bombs" that use conventional explosives to broadcast low level radioactive contamination over relatively short distances. Anybody with the wherewithal to build or acquire a missile delivery system may also have the means of building/acquiring enriched nukes.
On the other hand, if we are talking about a portable rocket grenade type of delivery system and not ballistic missiles, then yeah I'd say they would have to be considered very good candidate dirty bomb delivery systems. Again, though, it might not be worth the bother. It's easier just to leave a suitcase bomb at a target than to strap an equivalent onto a rocket.
I'm actually surprised (thankfully so!) so-called dirty bombs have not become a reality for stateside terror attacks. Low-level nuclear material is as close as the local hospital radioactive waste repository or chemical supply catalog (which pretty much anyone affiliated with a university or research institution can order from without question). I assume the bed material from next-generation pebble reactors would be ideal material, and there are I don't know how many stalled pebble reactor projects there are in Europe where material could be procured.
Then again, the bang just might not be worth the buck for potential terrorists. Small impact acts like a dirty bomb explosion are more aimed at striking widespread fear and terror (like anthrax in the mail) than really being out and out destructive, right?