Did you tell them why you want them to uncheck you in? If so, probably a mistake.
If you couldn't fly tomorrow, you can't be checked in and receive a credit for future travel. And if you couldn't get to the airport/gate, DL res would HAVE to uncheck you in. Though I hate to say it, let's hope this is not a "best in class" enhancement. NW has unchecked me in on two different occasions with absolutely no problems.
John
Same with me (on both DL & NW metal).