how can I connect my iPhone to a P.A. System. Is there an available adapter for this?

any minijack to normal jack converter like this

http://www.pchonline.dk/images/showpicture.asp?file=301/{8F4C7AA1-0143-40D0-887E-F1DE7644E905}_{D307AD5B-9B9A-4FFE-9473-916579B7754E}_40 -503-63.jpg&title=Adapter%20MiniJack-F%20-%3E%20Jack-M

would work

any cable with a minijack in 1 end and rca connectors in the other would work

http://www.ecogneato.com/images/support/stge/pabv/2/i1.jpg

the iphone4 minijack is like every other minijack equiped device it's stereo analog 2 channels and 1 ground

you can connect it to everything else analog 2 channels stereo input source in the world