If you check the PCA web site under GA 2005, you can read the various resolutions that will be coming up. That should give you a feel for what issues will be discussed.
In the past they have had a separate section for observers. You do not need a pass or anything to get into the observer section. The commissioners wear a badge to identify them, and only folks with badges are allowed "on the floor" of GA.
Can a non Presbyterian go? I would like to hear what Piper has to speak on. I would guess that since a Reformed Baptist is speaking then a Reformed Baptist could attend???