As a joint replacement surgeon myself, I get many requests for such letters from my patients. From what I hear from them, it really is a hit or miss and often depends on the individual agent. Implants are made of Cobalt-chromium or titanium.
Best way for now appears to be go into the body scanner or just preemptively mention you have a joint implant and ask for the hand held metal detector. There is even a downloadable TSA "notification card" but it doesn't hold any more weight than a letter. I believe this issue will continue to grow in the next few years looking at some of the projected trends in joint replacements as more and more baby boomers end up getting them.