Here is a post from the BC/YC FAQ Thread posted by TeenaS who is a greeter at BC.
Got some clarification on the new mugs.
The prices listed before are correct. A chip is on the bottom on the mugs. If you purchase a one day mug at 9 PM Sunday night, it stops working in three hours. Midnight is the cut off time ... doesn't matter when you purchase them.
The "length of stay" mugs are really 14 day mugs. Let's say you check in today at the BC and purchase the length of stay mug. It's good at any Disney resort (and now they are saying only resorts, not water parks) for 14 days. So you can do split stays, you can go on a 3 day cruise and come back and for 14 days, your mug works. If your reservation is for 17 days, you bring the mug back to merchandise on the 14th day and they will reprogram it for the next 3 days. They can "read" the chip that is on the mugs so they can tell the day you purchased it, etc. As far as loosing one, you MUST show your receipt.
You cannot microwave these mugs. It fries the chip. If you bring it back to merchandise with a fried chip, then you are out of luck, receipt or not.
There is also a regular old cardboard cup that can be purchased for a few dollars. It is good for 2 hours and/or 4 refills. It also has a chip on it.
The dispensers only dispense 12 ounces at a time and once you use your mug for a fill-up, you cannot get another fill-up for 2-1/2 minutes.
That should answer almost all the questions about split stays. The only thing I don't know about in regards to a split stay is if you are on the Dining Plan for the first part of your stay and then you have a room only for a couple days. The mugs are included with the Dining package so are they only good for the days you are on the Dining Plan?