From Downing Street:
The Cabinet confirmed that the wedding on Friday, April 29 next year will be an official bank holiday. The day falls at the end of Easter week and is the day running into the May Day bank holiday weekend. As a result, it will create two consecutive four-day weekends.
From Business link, the official government website, (via Brussels!):
The Cabinet has agreed that the marriage of Prince William and Catherine Middleton on Friday 29 April 2011 will be marked by a public holiday.
However, the announcement of an extra bank holiday does not increase any entitlement to holiday under the Working Time Regulations, so whether an employee will benefit from the additional bank holiday will be entirely dependent on the wording of their contract.