officially, you are allowed to only bring ID. Everyone else's mileage may vary.
Why do you people ask these questions? You don't need a watch -- there is a clock on the screen. Unless you are planning to cheat, you don't need anything except scratch paper and pencil, both provided by the proctor. If you expect the worse case you will not be dissapointed or unpleasantly surprised.
I think your time is more wisely spent worrying about the content of the test.
Former Senior Instructor, Manhattan GMAT and VeritasPrep
Vice President, Midtown NYC Investment Bank, Structured Finance IT
MFE, Haas School of Business, UC Berkeley, Class of 2005
MBA, Anderson School of Management, UCLA, Class of 1993