The video asserts that Joyce Lin (the aforementioned person acting like a five-year-old) was within her rights to not produce identification and walk away, but this may not be true depending on New York law. Nevada has a law that requires suspects to identify themselves in certain conditions, which was upheld by the U.S. Supreme Court in Hiibel v. Sixth Judicial District Court of Nevada (542 U.S. 177, 2004). If New York has a similar law, Lin was required to identify herself.
UPDATE (December 17, 2008): A Critical Mass bicyclist knocked from his bike by an NYPD cop in a similar incident in July, caught on video and viewed over 1.8 million times on YouTube has been cleared, and the cop indicted, stripped of his badge and gun, and assigned to desk duty. The NYPD officer in that case was Patrick Pogan. Sometimes flagrant police abuses do get punished, but it's a pity they often have to be caught on video and seen widely for that to happen.