Hi there: at the request of some users on #emacs, I've patched sh-script.el
to do the following:

   `command args...` is highlighted in a different face than normal strings

   $(command args...) has command highlighted in the same face as above

   "some text $(command "moose burgers")" is highlighted correctly, at least
   in that the "" inside the $() do not terminate the "" around it.
   this new support copes with nested $() and () and `` subshells within
   the "$()" construct.

   "`command "an argument"`" is similarly correctly handled (well... more
   correctly than before, anyway)

The changes are not invasive: there's a new item in sh-font-lock-keywords-var
for the bash shell, and the syntactic fontification variable likewise
has a new entry (with two support functions and a new face defined).

Hopefully this is of interest and can be included in the next version
of sh-script.el

[ The diff is against debian's emacs-snapshot package v 20060202 ]