Next year, after I've settled in at my new West Harbour City condo, I may upgrade my 40D to the 7D. As a special treat to myself, I may also buy the first, and probably only, 'L' lens in my lens collection: the 300mm f/4.0L IS for C$1,400. This baby is equivalent to 500mm on a full frame camera! (The 500mm f/4.0L IS sells for C$7,000, a difference of $5,600. This monetary difference could pay for a vacation in Japan and Australia!)
Not only is the 300mm a lot less expensive than the 500mm, it's also a lot lighter -- 2.6 lbs vs 8.53 lbs. This hernia-inducing 6-pound difference makes using the 500mm a terrible burden.
Now you understand why the 7D, with its 1.6x crop factor, is so attractive. You can achieve much longer focal reach for less money and less weight.
Here's a review of the 300mm f/4.0L IS.
To recap, the choice of 7D over 5D Mk II saves $1,000. The collateral choice of 300mm over 500mm saves $5,600. Total saving? $6,600. Don't we feel richer now?