[ChadB]

The big problem with that is that you'd have to use the same

  really_long_random_string

to register for every gravatar-enabled site you want to register for.

The only benefit I can see of this is that you could easily blacklist emails destined for that address.

[pyre]

Doesn't have to be a really long and random string. Just has to be something that can't be easily deduced from your name and/or username. This might not defeat rainbow tables, but I think there are probably easier ways to harvest email addresses than to generate rainbow tables and troll for gravatar URLs.