Brendan opened a Firefox ticket this morning to make line-number mapping possible, and that's really the biggest outstanding issue for debugging, since the semantics are otherwise one-to-one. In fact, it would make <script type="text/coffeescript"> tags a lot more fun to work with than they already are, since debugging through an "eval" would be far less painful if exceptions pointed back to the proper position in the original source.
We could go one step farther than line number mapping and just define and implement a standard for full-on support for original source pointers. Then we could implement any number of langs using minified Javascript as an intermediate language.
The bug I filed based on Jeremy's suggestion is about source coordinates, not line numbers. We are way past the ancient mistake of numbering lines but not columns, which bites minifiers all the time.
https://bugzilla.mozilla.org/show_bug.cgi?id=618650