I have just installed a fresh Xubuntu 12.04. The Gnome keyring only seems to work if I select "Load runtime environment for GNOME at system start" in the settings menu.
Specifically, I am using a secondary SSH key stored in, say, ~./ssh/id_rsa_2. Without the above option actived, I have to re-enter the SSH keyphrase every time.
However, I don't actually want to load a full "runtime environment for GNOME" on startup -- I just want the keyring to work, nothing else.
On my Unity machine, on the contrary, this all works perfectly fine and hassle-free out-of-the-box.

Thanks for your help!