I've had more than a few Dell laptops and we currently are running two. In the past when I've complained about HDs, they have offered to replace them free, while under warranty. They send me a new one, I send the old one back. Period.
I quess it's not really to your benefit to have a tech come over, I'm sure you know HDs are easy to replace yourself. Dell, in my experience, doesn't make a big stink about small items (battery packs, etc) as long as they get the defective part back. I doubt you're in violation of anything if you erase it.
That said, one of two things will happen. If the tech is able to 'fix' your HD, no harm, no foul. If not, you'll get either a new blank one, or one loaded with Vista and possibly mucho crapware. Of these latter two scenarios, I rather have a blank one and reinstall Vista anew.
In any case, backup what you can and please do stop by tomorrow and fill us in on how it went.