I've received the following email from TfL:

"From Monday 14 April until the end of June 2008, while major
escalator refurbishment work is in progress, Clapham Common station
will be exit only during peak times:

Monday to Friday 0700-1100 and 1600-1930

Please use nearby Clapham South station for the Northern line and
Stockwell station for the Victoria line during these times."

Haven't seen it mentioned anywhere else (other than under the broad
and not-very-detailed description of "escalator works"), so I thought
I'd post here.