Holmes4 is absolutely correct. Dell will most definatly bill you for the drive if you do not return. You'll use the same box that the replacement comes in. I'm surprised that wasn't explained over the phone/chat/or whatever means you used.
The return period should be 10days, if you find that's not enough time.. ie: you need to do data retrieval or wipe it clean or whatever, just call up Customer Care, and they should have no qualms about extending the time you have to return the drive.
Another note about hard drives though... and I don't know that it'd be available to consumer systems.. ie: would/is available for the Latitude 2100.. There exists a contract type called KYHD (Keep Your Hard Drive), the cost varies per system, the age of the system, and some other factors like what company number you are.. 4, 8, 12, 22, 28, 40, ect.. But if you are ever concerned about your data, its a nice thing to have.
The last time I quoted someone for that contract type was for an LatE5500, the KYHD contract cost was about $18.00. So if that persons drive were to fail and we were sending a replacement, there's no need for them to send the old drive back.
As for the reason Dell needs even broken drives back? ..Dell sends those drives back to their respective manufacturers, so they can get credit for products they paid for that didn't work.. in the end, it helps to keep costs down for everyone involved..