In Firefox, viewing the original SASS line number directly has been possible using Firebug and FireSass. Similar, or even better functionality has very recently arrived in the stable release of Chrome. I just tried this out and it works wonderfully. See here for instructions:

How to make Chrome understand the SASS/SCSS in your rails app

Don’t worry about the Rails stuff, these instructions work just as well for stand-alone SASS. If you’re using Compass like me, you need to set sass_options = {:debug_info => true} in the config.rb file of your project.

If you look at your css files after doing the above, you’ll see they are quite a mess. From a comment by Paul Irish in the above post I learnt better stuff is on its way thanks to the upcoming SASS 3.3, and Source Map support in Chrome:

Debugging SASS with Source Maps

Exciting times!