March 2012 was BT Openreach's holding date for planned cabinets that don't yet have specific activation dates scheduled. Last week they bumped them all to December 2012. This has absolutely no effect on when they'll be delivered; at the point an activation is scheduled the holding dates will be replaced with something more realistic.
In terms of cabinet to activation; three cabinets were installed near me in June. One went live in August, another went live in October and the third is still without power. The head-end at the exchange went live in 2010.
Without insider information, no-one can predict!