This following experience isn't DL-specific, but apparently some types of delays result in an pre-set delay initially being set that might not match up with the actual delay at all. So I'm curious if this is true for DL as well.

I am in SEA right now where it's been cold and snowy the past couple days (actually, I like the weather, except that Seattle has no infrastructure to deal with snow and ice.) Two of my friends were returning SEA-DTW yesterday morning on the 0845 NW non-stop, so I figured I'd keep an eye on delays for them since I was up late. (The joys of occasional insomnia...)

Around 0730, their flight suddenly showed a mechanical-caused delay until 1500 on nwa.com. Called my friends and told them to call NW ASAP to get protected on another flight since holiday travel had loads very high. They were already at the airport and apparently everything looked fine there. Flight was still showing the same at 0800 when I finally slept.

The flight actually left SEA at 0853 and arrived 13 minutes late. One of my friends found out that the aircraft just needed a new fuel filter, and that apparently 6 hours was a "standard" initial delay. Nwa.com has a "Delayed due to non-scheduled maintenance" note for that flight.

I would assume DL would have accomodated the OP under the flat tire rule at least since it would've been within 2 hours, and if not would have done so anyways from their published mis-information. Personally, if the delay is around 90 minutes or less I just go to the airport as planned. (Or get on an earlier flight if I can.) If it's longer, I call and try to find out the real story to decide whether to leave late or not.

One more point here that's a dup of a previous post but relevant... I was flying IND-ATL-BOS a couple weeks ago. The weather in ATL was bad, but my flights were still showing on-time as we were nearing the airport (yay for EV-DO.) Since I had a 37 minute connection, I called SMS and asked them to protect me on the ATL-BOS an hour later, which they kindly did. Thumbs up to DL for doing that even though everything was still showing as on time! (ATL got even worse and SMS rerouted me IND-CVG-BOS in the end.)