Well, I don't see how this can be useful. Human brains are terrible in remembering faces, but not in remembering colors. Actually, comparing the icons (to find out if they are the same or not) is quite hard. Have you tried it?
while i was developing this approach i tried lots of ideas. it's actually quite hard to get images with (1) a wide range, to avoid "collisions", (2) attractive designs, and (3) coherent forms (that don't look like a random mess, making them easier to remember). one of the things that helps a lot in making these more distinctive/individual/memorable is the diagonal symmetry. another is the generally consistent hues within any one icon.
stuff like these:
http://lookslikerain.deviantart.com/art/Drama-Queen-13698449...
I think you need to emphasise this point more in your 'about' page. I skimmed through your about page and came away with the thought 'I don't think I'd want to replace my personalised avatar with something like this - it's much harder to identify me through this than a photograph or illustration'. But as a replacement for a generic avatar - that makes sense!
This is a matter of personal preference. The more people you ask, the more opinions you will get. I could chip in mine, but that would serve no purpose.
You could see statements like these as a form of libel/defamation. While the statements are not necessarily false, they may cast a negative light on whomever is generating these images (in this case, gravatar.com).
I would try to avoid these statements, and simply go ahead with your project. And let the people decide for themselves which one they prefer.
There are thousands or millions of FOSS projects that were started because the existing ones are bad, and they've all said that they were started because the existing ones are bad. It's just the way it works.
Now, if both he and his competitors were serious companies with a handful of employees that were charging money, then it would be less appropriate. But they aren't. His project was created because the competition's projects are ugly, and he should be able to say that.