They will only allow you into a park in which you have a dining reservation if the ADR was booked on a credit card. So, if you have an ADR at CRT or Akershus (those are the two I’m thinking of atm… maybe Bistro de Paris), then you can still get into the park after it has reached “capacity.” Any other ADR, and you’ll need to go to guest services to rearrange your ADRs. They’re really nice about it and try to accommodate you as best as they can. We learned this out the hard way when we had an ADR at CP in MK, but the park was no longer accepting guests.
As for resort guests always being allowed into the parks, don’t be so sure. It depends on how you are getting to the park. If you are a resort guest that is currently at Epcot, then they will not allow you to take the monorail to TTC to switch to MK (craziness! We learned that out the hard way, as well). But, they will allow you to go to TTC and switch to the resort monorail, so you could then get on the resort monorail and get off at MK. Whether or not they would accept you at the gate, idk because we haven’t been that persistant about it (if the park has reached capacity, you really don’t want to be in it, anyway, unless you have an ADR).
You should really just be careful and make sure that you are in the park an hour before your ADR, and if you are going to MK on a day that is typically crowded (any day the week before/after Christmas, week of Thanksgiving, week before/after Easter, and week of Fourth of July), be sure to arrive early in the morning. If you want to be able to experience attractions, it’s best to arrive at opening time.
MK is really the only park you ever have to worry about reaching maximum capacity. Epcot is large and can accommodate many people. AK is also large, and DHS isn’t nearly as popular as MK. I have only seen MK hit maximum capacity recently. The last I saw AK hit maximum capacity was the year it opened (we were rejected :frown:).
Also, MK typically reaches maximum capacity between 10:00 AM and 12:00 PM. By 4:00, the crowds have usually thinned out, and they begin allowing people back into the parks. So if you left the park around noon and came back around 4:00, you shouldn’t have much trouble getting back in.