Not exactly what you asked but I thought I'd mention, I use this for dynamic css and love it.
http://www.misuse.org/science/2006/09/26/dynamic-css-in-ruby-on-rails?story=20060926084103529
Basically you create a controller (rcss_controller) and your css file(s) are just templates which can now contain ruby code, instance variables, etc.
linoj
Another good reason to use conditional comments is that you aren't preventing yourself from ever being able to use page caching... Then the html output is the same regardless of the UA. Of course this doesn't matter if your pages are all user-specific anyway, but some of the cms stuff I've built does very well with page caching since everyone sees the same page.
Matt