You like to have a gravatar close to each comment of your blog but… what about an icon near your pingbacks/trackbacks? The solution is: use the favatars plugin.
Unfortunately, it doesn’t install on WordPress 2.5.x, so I’ve just opened a ticket (with a patch) that fixes this problem and that speeds up its one-time favatising SQL query (making it loops over pingbacks/trackbacks only).
It seems that its author doesn’t maintain it anymore so I’ve just put here the modified version of it (with my patch applied).

[...] Go to the author’s original blog: favatars fixed for WordPress 2.5.x [...]
are you willing to take ownership of the plugin development ?
:-D
kOoLiNuS: mmm… maybe. :) A feature that I think it should have is caching (avoiding to http_get remote favicons).