This is something I only just realised after wondering for ages why standard library calls (sin, cos, clock, etc) weren't working.
Nothing in the docs mentions you need to call sq_pushroottable(vm) before calling sqstd_register_mathlib(vm) (etc). I only noticed it in some code someone posted in another forum post, and it worked :) Perhaps a note could be added to the help docs (sqstdlib2.chm) to save other people struggling with the same problem.
Er, otherwise this is my first post and I just like to say how easy I've found Squirrel to use. Thanks Alberto!