For our half yearly exams I had my phone on silent in my pencilcase. A certain person, not relising I was in the exam let it ring out like 6 times in a row.... the noise it made vibrating against all my pens and pencils was enough to make a few of us giggle, but the teachers didn't notice...
so I really don't see the prob with having it on vibrate, especially if you are expecting an important call (so long as its not against a hard surface!)... if you keep it in your pocket you can feel it and it makes no noise whatsoever, so you can excuse yourself and answer it as you reach the door (I do this in the cinema).
oh, and if I sit my phone on a table, it doesn't make much noise when it vibrates, it just dances its way off the edge and smacks into the floor
but I wouldn't really think many people would put thier phone on silent and sit it on the lecture tables... thier barely big enough fior a notebook, so I wouldn't think there would be room for a mobile as well?
my 2c --> if you *need* to have it on, keep it in your pocket, and don't be the noob whose full volume, polyphonic star wars ring tone interruprts the lecture