My, they have been busy. Officially, the formal agreement must be signed by Gordon Brown while he is in China for the closing ceremony of the Olympics. I think that will be the next step in the process.
As much as it is breathtaking to visit the edinburgh zoo website and see the amazing species dotted across their pages, and as exciting as it will be to see giant pandas in the UK again, western zoos really aren't needed to house, breed, or fundraise for this species. China is big and ugly enough to conserve it's own wildlife. I don't believe this is a conservation decision on the part of Edinburgh zoo. Sure, they will raise a huge amount for the general work of the zoo, but I guess that's where the argument starts as to whether it's acceptable to use animals in that way.
As for Mercedes, she's not exactly ancient yet, I'm sure she'd survive crate training and/or sedation to be moved to a better enclosure, either at HWP or Edinburgh. I think the main issue the activists have with the zoo is the reversal of the pledge not to replace her when she dies. I personally am torn, the zoological society of scotland is building a more dynamic and interesting collection of species than I have ever seen in my lifetime in the UK, but I think their priority should have been a better polar bear exhibit sooner.