I pulled the ripcord on my 401K and moved mostly out of the market. Transactions don't process until the end of the day so I was watching the market like a hawk all afternoon.
I screwed up by being too conservative early on, but I gained a lot by holding my breath and riding the market down through the fall of 2008 and moving my more conservative investments into the market in April of 2009 just missing the bottom. Many people I know lost 25% or more of their retirement savings by panicking and pulling out of the market when it started down, then waiting until it was really climbing before moving back in.
I hope I'm not missing a big top, but I'm only two years max from retirement and I couldn't see being fully exposed any longer.