Various websites , including Extron mention 15ft, 45ft and 75ft as the possible limits of HDMI cable length, based on quality of cable, amount of data sent and type (power specs) of the graphic card of the device sending the signal.
Aside from the cell phone-caused speaker interference, there are additional sources of noise/interference in an audio system, including mic feedback. Here are some more resources that describe ways to reduce feedback:
Projector Central to help you pick a projector: http://www.projectorcentral.com/
Calculate screen size and projector distance based on seating arrangement and aspect ration: http://myhometheater.homestead.com/viewingdistancecalculator.html
Figure out aspect ratios: http://en.wikipedia.org/wiki/Aspect_ratio_(image)
Here’s a nice computer (aka CPU) mount:
Freedom podium/lectern with Freedom link cart (actually the cart may be more useful for us than the podium!):
For example we could lock a laptop to podiums where there isn’t enough space to attach a swing arm and desktop mount:
Kensington Laptop Locking Station ($80).