@ dhays
Under US libel law, the truth hardly matters. There are all sorts of additional requirements for a successful prosecution, especially where public figures are involved (details are easy to find, so I shan't repeat them here). Even if some (or all) of Allen's comments were provably false (and apparently some have already been disputed by third parties involved), it's very unlikely that Gates would be able to win a libel case under US law.
The enormous bias in US law towards defamatory writers/speakers (and I'm not implying Allen falls into this group), as opposed to their victims, probably explains why the US media tend to be full of malicious lies about public figures. Libel/slander isn't a crime under US law (or in common law jurisdictions generally), and US law is so biased that suing for libel/slander there usually just brings more attention to the defamatory claim, increasing rather than diminishing its impact on the victim's reputation.
I'm not, by the way, necessarily defending UK libel law (also common law), which has a bias towards the side with the most money, but on the whole is much more balanced than US law (which basically allows lies about public figures to be published with impunity). I think the classification of defamation as a crime is the more correct approach than the common law one.
