They should really sell computers with two hard drives in them for just this very reason.
That's a good question. If you don't have their on-site service, and you're directed by a telephone support person to open the computer, then the answer would be no. OTOH, if you do this on your own, it might... But then if you don't go after a warranty replacement of the hard drive, Dell would never know...
Then again, I don't know if doing an upgrade to the computer (adding a hard drive, DVD-ROM, etc) would void the warranty or not, although the new parts would not be covered, unless you bought them directly from Dell.
The example I gave was for a specific situation, where a client didn't feel comfortable with running the diagnostics themselves, and decided to have a technician come out, at their expense. From the errors that they read off to me over the phone, it was obvious that the hard drive was failing, but Dell wouldn't accept that. In that case, it would have been cheaper for them to buy a new hard drive than to have me come out and run the diags.
Mark