The British government is planning a celebration for the 150th anniversary of "Big Ben" in 2008, so they have just began (today) a scheduled 5 – 6 weeks maintenance project.
Therefore, if you are planning a visit to London and specifically "Big Ben", you may want to rethink your plans.
Fortunately, there are many other great tourist spots in London. Take a look at my Flickr photo album for London.