1) - UVC compatible webcams SHOULD work fine.
2) Somebody else here on the forums today showed his RPi connected to an Arduino UNO (over USB) sending commands to the Uno. Not sure about the Arduino dev environment though - it's Java based, so it probably will work, but I don't know what it uses under the hood for compiling and uploading to the Arduino.
3) Eh, depends largely on there being drivers. If drivers are required to use it on other systems, then probably not a good chance of it working (unless you find some other ARM Linux drivers for it... so MAYBE, but don't get your hopes up... This is one of the reasons I prefer Hotspots over plug in USB dongles..)
4) GPIO pins are basically similar to the Arduino's pins, except they are at 3.3V (rather than the Arduino's 5V). Check the wiki to see which pins do which special functions. Currently if you want to interface with the pins you need to do bit-banging in language of your choice...