As you can see on my blog, all Gravatar images lost the transparent backgrounds, making them f* ugly. At first I thought it was some kind of error on my browser (Debian unstable, you never know..).
But then I experienced the same symptoms on other browsers. Google pointed me to this blog post that explained everything: they switched from Rails to PHP and now they can’t support PNG with alpha channels (this is actually a joke, read the article for the true reasons).
Although there are many comments on the blog entry begging for PNG to come back, it doesn’t seem that Gravatar will take this step back. So for me, they now suck.