HERE is the link for those who don't want to type all of the "dubyas".
Michael, it's not a bad place to work with. It's a shame you couldn't put some walls up since part of it is finished already.
I doubt the pipes will give you any trouble. Usually it's the ductwork that is the problem... but I don't see any in your photos (you must have radiant heat?). And like Chris said, if the pipes do rattle/ring, that's an easy fix.
As far as sound being "sucked" into empty spaces.. you're partially right. The right/rear surround looks like it'll be hanging in open air, or at the very least against the "I" beam. This means that speaker will be operating in "whole space" as opposed to a normal "half space" against a wall. So, you'll probably notice less sound coming from the right rear since some of its energy is being wasted to the space behind it. However, don't be discouraged, any surround receiver worth it's meddle should give you the option to adjust the individual speaker volume levels. In short, you'll just wind up turning the right rear up a little louder.
Another problem you might have is reflections coming off of that tile floor. It might be a little "echo-y". You have the right idea by putting a piece of carpet down.
As far as soundproofing goes... when I set my stuff up in my new unfinished basement, I was really impressed on how little sound actually made its way to the second floor. Any sound that did make its way up there was usually through the ductwork. Of course, I'm not a "reference level" kind of guy... but I do enjoy my bass.
I would say go ahead and set it up and see how it goes and deal with the problems as the arise. Since it sounds like you're not doing anything permanent, you can fix things after the fact.
P.S. Welcome to the forum!