I'm not sure how best to make it part of the site sign up process on the server side. The other hassle is you now really want to also link mobile platforms, so unless you're all apple, it's kind of tricky. You could possibly do a web-based password manager instead of something local (there are a few options), or if you're all-apple, 1Password (which doesn't even work perfectly on iOS).
The other way is to just move everyone to Facebook Connect or other third-party identity services, but there are a bunch of problems with that. If there were a single-signon provider which were only single signon, vs. profiles and all the other stuff, it would be more acceptable, but even then, just linkability of all your accounts is a security and privacy compromise, as well as being a single point of attack and failure.