Simply put, by default each vCPU you configure a guest with is presented to the Guest OS as a socket. Various benchmarks suggest there's no performance difference between (for instance) a four single-core socket VM vs a single four-core socket VM, which is fine and dandy.

Unless you use fancy software that does CPU licensing.

This post has a great explanation, with anecdotal evidence that you can fix this, albeit expreimentally, in ESX 3.5u2 and beyond.

Also, something I found today that is the (now second-) most useful thing I'll find all day:

Access to Sysinternals Tools without installing them!