It seems someone is going round messing with billboards in London by chopping their heads off, check it out on Flickr.
To anything that messes with the ubiquitous capitalist propaganda I say well done. I recently heard of another good idea along these lines; sticking 'actual size' stickers on billboards with 15 ft. long tubes of toothpaste on them. Very good, can't find the link to that one though. Why don't I just do it, yer anarchy! (I'm probably not going to do it as I'm a bit of a coward and kind of at the office right now).
Found at 30gms, who I think may have coined the term, The Shoreditch Slasher, which works. Go Slasher.
Update: it turns out the actual size stickers are from the xkcd store, where he is trying to sell them for $5, capitalist scum!
http://the-real-linda.blog.co.uk/
05/01/08 @ 00:19