When I signed up for Ohloh I didn't have a Gravatar, but I like my Ohloh account so much that I went out and signed up for one. This was several months ago now, and I don't have an icon here yet... does Ohloh update from Gravatar at any point?
I kind of assumed that Gravatars were dynamic and just linked from their servers anyway. Must be wrong.
I'm not sure what the current state of Gravatar is, but in the past it has been known to take little while for new Gravatars to appear.
We don't cache the images locally -- they always come directly from the Gravatar servers.
If it doesn't appear after a day or so, one thing that I know has bitten a few people in the past is capitalization. Gravatar keys are case-sensitive, so make sure that the email address you gave to Ohloh is capitalized the same as the the one you gave to Gravatar.