Types of Bowing In
I, too, can't say I've ever heard of "O-sensei/Sensei/Shihan ni rei" in aikido. I will say that we used to do the whole reading of the five dojo kun, "Sensei/So-shihan ni rei" business when I was doing karate, though, so it may not be too foreign.
As far as the types of bowing in that I've seen done, it really does vary from dojo to dojo. Here are some (certainly not all) that I've seen:[list=1][*] Bow once, clap twice, bow again. Teacher turns around. Bow to teacher.[*] Bow once. Teacher turns around. Bow to teacher.[*] Bow once, clap four times, bow again. Teacher turns around. Bow to teacher.[*] Bow to the shomen. Bow to the instructor. Bow to each other.[/list=1]
We do 1) at our dojo. If I remember correctly, we did 2) at Aikikai Hombu Dojo.