Canon said the two are running side by side at the moment, but new releases will happen at faster rate - yearly rather than 18 month intervals.
So, when the 50D is replaced, my predictiton is the 60D will take place of the 50D and the 50D will drop to where the 40D is now and the 40D will be discontinued.
This won't happen until October 2009. The 40D remains a fine camera and will do so after that.