This is something Zen are aware of and we are looking at being part of the BT trial , however initial indications lead us to believe this will be geographically limited due exchange capability, we will post more details as we receive them.
My connection is stable, although I only opted for the 2mbps upstream.
40mbps is a world apart from the almost 2mbps I got from ADSL with Zen. I am happy as can be with these sort of speeds, but I'm sure I wouldn't complain is somebody decided to double the capacity for no extra charge...