Does Changing Your SSH Port Lower Your Risk? (2008)
danielmiessler.com
danielmiessler.com
Obscurity will (at least in the case of an SSH deamon) not make an attack any less likely to /succeed/, but it will make it less likely to /happen/.
Note that what he observed were automated, scan-everything-you-can, undirected attacks. I would presume that changing the SSH port will do very little against a targeted attack.