Here is a good place to start on the speaker placement.
As far as the conduit goes, you shouldn't need it for speaker wires, but definitely run it for the projector wiring. Use at least 2" PVC for that. Make sure you don't put the outlet too close to center. Depending on where your projector ends up, you don't want to have the conduit outlet in front of the PJ.
As far as the placement of the projector, that's really dependant on the actual projector you choose. The throw distance on projectors can vary tremendously. I would poke around
Projector Central and look at some of the recommended projectors. They will have throw distance calculators that you can use to see what the different throws are in relation to your room. To do so, you will have to have an idea of what your future screen size will be. You'll also have to consider whether you want a 4:3 PJ or a widescreen PJ. This will affect the type of screen you'll need. Also, there is a vertical offset with projectors (top of projected image is lower than the lens vertically if ceiling mounted). This shouldn't be too much of an issue, unless you are mounting to a large soffet that is significantly lower than the ceiling. This really only plays into how long your PJ mount is.
As far as cabling to the projector, that's really dependant upon your applications. I presently have a 25' svga and a 25' component cable pulled. The svga cable will go away once I have my receiver that does component switching. If you think you'll need it, you can pull an s-video, as well. However, this type of application begins to beckon for a receiver that does video upconversion, which then limits your needs to a single component cable. Just make sure you pull through extra strings for future pulls. It's a pain to pull cables through once another cable's in there, so you end up pulling one out and then pulling both back through. Not having extra pullstrings available could make it more difficult.
Another thing to consider is where you put your speaker outlets for the fronts and centers. Make sure you don't put them centered on the screen wall (made this mistake). They will be a distraction. The same goes w/ the power outlets. Both of them should be offset to where they'll be behind the speakers, etc.
Good luck.
