AFAIK, there isn't one that's accessible from the external part of the case. Usually the most sure-fire software method is to hold the mouse button down during boot (reference). I believe that anything better than that involves actually opening the case to access the optical drive mechanism.
I too believe that there is no direct, external mechanical way to trigger a disk eject. I've been traumatized a few times with a CD or DVD that I had a very hard time ejecting on my old iMac G5. I don't recall what eventually worked, sorry.
Thanks for the replies, luckily I don't have a disc trapped in a drive. I mounted an old Combo drive (Matsushita) into a $20 external USB 2.0 case (works fine) but was wondering if there was a manual eject switch/button "just in case."