Originally Posted by BotB

The drop off / pick up point is by a bus stop (for many different hotel buses and mini vans) just to the side of the altar of the fatherland, (aka Altare della Patria (on Via del Teatro Marcello)

Map location (courtesy/copyright of Google maps):

You can find out the timings and a few caveats regarding the shuttle bus that you should know about in my post upthread (link here: http://www.flyertalk.com/forum/25967946-post439.html

The bus worked great for me but they do state that they are not responsible for delays, full busses (so no seats available) or for not showing up...so caveat emptor if your plans all revolve around getting that bus to make the hotel choice a better deal...it should work out but have a backup plan or consider how you would decide if the bus is not running/is late, etc...so you enjoy the time in this great city...