The new tiger enclosure is still going to happen but they've put it off till 2012... I think because of getting the funding together; it's such a big project. In the meantime they are going to redevelop and enlarge the current penguin pool, not the Lubetkin one obviously, but the Barclay Court one; it will be much bigger with better viewing facilities and there will be a glass wall to the pool so you can see the penguins swimming about underwater. I gather they will be demolishing the old 1869 "parrot house" to the side of it, which has the schools' lunchroom at one end and the ground hornbills in front. I'm quite surprised because I thought it was listed, but obviously not, it's just old! That's all I know for now, but will post any news I hear.
The other change is that a new exit is to be built behind the main gift shop on Barclay Court, so that visitors go through the shop (that old trick!) on their way out. So that area of the zoo will be pretty chaotic for a while... this is all imminent, I think.