vim spell checking - comments only in LaTeX files

The latex ft plugin conveniently defines this behaviour.

SpellChecker : Spell check text including LaTeX documents

Using latexmk, vim spell checking and vim latex-suite

There is an option that appears to come close:

:syntax spell [toplevel | notoplevel | default]

Update

Also

:he ft-tex-syntax

has very useful tidbits, like

Don't Want Spell Checking In Comments? ~

Some folks like to include things like source code in comments and so would
prefer that spell checking be disabled in comments in LaTeX files.  To do
this, put the following in your <.vimrc>: >
      let g:tex_comment_nospell= 1

You'll have to figure out whether you can use that/extrapolate from there

I had the same problem (VIM 7.3), but this post at the vim-latex-devel mailing list provided the clue. To get the spell checking working, I had to put

syntax spell toplevel

in my ~/.vimrc, but it has to be declared after

syntax enable

or

syntax on

for it to work.