Node uses Google's V8 virtual machine, which they built for Chrome. JavaScript is single-threaded, so yes, unless you spawn a child process it'll be limited to a single core.
Only implicitly: no threading model is documented in the ECMAScript Language Specification [1], and no widely-used interpreter implements threading. The new Web Workers specification adds something like threading to JavaScript in the context of the browser [2], and Node intends to add support for this on the server as well [3]. Have a look at [4] for a nice article on threading in JavaScript.
