It's a blow to the reputation of Vista, but raises my respect for Dell. When Vista came out, they switched all of their consumer computers to ship only with Vista (only business computers still had the choice between XP and Vista, since corporations are often tentative to upgrade early). But there has been enough consumer demand for XP that Dell once again gives their consumers a choice between XP and Vista.
This doesn't really effect me at all, since I'm not in the market for a Dell at the moment. But it does give me more respect for them as a company, and increases the chance of me recommending them (or, rather, continuing to recommend them) to anyone I know who is in the market for a PC.
Good on them.