Plans are afoot to link London to Paris with a series of cycle paths. The idea is to have it ready for the 2012 olympics. (Update: 11th August 2011 - it's not looking like this'll happen in time as a continuous, well signposted, off road route. However, bits of the route are in place and it is cycleable.) I plan to cycle to Paris on these paths, when I get a window of opportunity.
(The 'Franco British Cycle Plan' is now called the 'Avenue Verte')