I really love this plugin which makes replacing the standard Gravatar implementation so easy :-)
Just while using it, I noticed one thing: Because of (inteded) browser caching, an updated avatar of a user might not show up (immediately) if he deletes the old one and uploads another one. This is due to the selection of the uploaded filename ($user->display_name . '_avatar' in the function "unique_filename_callback").
I would propose to add a (short) hash based on the current time() value - this would make all browser to load the new file immediately!
Thanks for considering this addition in the next update...