On all the Northwest flights I've been on, an actual distance was read...

... however, on all the Continental flights I've been on, the FAs usually say, "... and our OnePass (and WorldPerks) members will be earning valuable miles on this flight."

In one perspective, it's nice to keep the customers informed... on the other hand, if FAs don't know the difference (rules of WPs, min. distance, EQM, etc.), I think CO's way works best.