No, of course not. They will just continue selling XP until the climate changes. Vista becomes decent or people decide they want to pay for crap. It doesn't mean they're dumping Windows, that would be crazy, their whole business is based on it.
What it means in practical terms is that as the first OEM they are saying f off Microsoft we'll decide what we're shipping. This is risky, because their income is based on MS' "subsidies", basically they both need each other.
If Dell decided to just sell linux they would lose a lot in sales, it's not worth it.