I'll add that two years ago when I (re)started FitDay my knees were in constant pain--it was pretty much all I could do to stand up out of my chair and going up our stairs was a slow, laborious process. Now I can run up the stairs pain free and my lower back issues have gone away to boot. As you point out, taking any amount of weight off can do nothing but help the joints.
47 M 5'8"
May 4, 2010...... 440? lbs. (Start FitDay Size 60 Jeans)
June 19, 2010.... 393 lbs. (First Weigh-in)
June 19, 2011.... 229 lbs. (164 lbs. gone in one year :-)
Current Weight... 185 lbs. (Size 36 Jeans)
Next Goal 169 lbs. (07/04/12)
Ultimate Goal 165 lbs. (12/31/12)
The best exercises for weight loss are Fork Putdowns and Table Pushaways.