I like the idea of having to leave grandma's because she wet her pants.  I might have to use that one -- thanks for the tip.
Today, we're giving the underwear a rest.  Everyone involved was getting frustrated and it didn't help that DMIL has been telling me to back off -- she watches DD two times a week and doesn't exactly stick to the routine I've set up at home, much to my dismay.  "I've raised 4 boys!" she says.  I think it's fabulous she's raised 4 children, but she hasn't raised mine!  DD is WAY different than her boys, even she admits.  Can you tell I harbor some bad feelings about this? 

I leave in about a week for a 5-day out-of-state trip for work, so I think I will start it back up in a couple of weeks.  Give DD a chance to cool down with no pressure.  And give my DH a break the week I'm gone! 

Thanks for everyone's advice.  If you have any other ideas, I'd love to hear them.  This potty business is the pits!