... it makes them look rather foolish announcing new exchanges when unable to complete current ones within 6 months of original schedule
On the face of it, this is perfectly true - you have to ask why BT are adding new exchanges into the 2011 schedule, when the ones already in there keep being put back.
But you also have to consider that the roll-out has a variety of dependencies - and those could mean that the overall roll-out can move faster, with more exchanges in parallel, even if some of the current ones are seeing delays.
For example, having spare engineers sitting idle in Lancashire isn't going to help get London rolled out faster, so you might as well set them to work in the North-West. Or if the bottleneck is in training engineers, then you have to keeping feeding people in where you can, and have some exchange areas for them to work on as soon as they're ready.
Or perhaps they've learnt that ordering power connections in Birmingham takes 3 months instead of 1 month - so they choose to start some extra exchanges early to take account of this. Maybe they'd choose to install cabinets first, well before they start on the exchange & backhaul.
Perhaps they're anticipating more snow next winter, and need to start lots of exchanges now, doing outdoor work, to give them more indoor work in the winter.
Who knows, but its a complicated project to manage, even if the technology itself is now straightforward. The trick is getting all the people being as effective as they can, no matter what delays the outside world throws in the way.
What I like is that they've now separated the list into a "coming soon" and a "bit further away" section. I think they're doing that in response to the frustration they've been causing, and they're trying to respond by being more accurate when they can, and more vague when they can't.