Yes it does void warranty. but once you unroot using superoneclick or flash a Stock rom available on samfirmware.com for i9003 u will have ur warranty back.
Yes, u can flash cooked rom..but therz no cooked rom for the device from any developer yet. Roms of other galaxy S series model wont work nd can brick ur device. Further, there is no custom recovery for I9003 model from any dev yet..so u justhave to wait untill sumone comes out with a recovery