Originally Posted by psili

Use a server side script to write the .js include:

if($_SERVER["HTTP_USER_AGENT"] == "so and so")
echo "this.js";
else
echo "that.js";

Or, have your .js file alias to a server side script that does the same thing internally.

Otherwise, I tend to agree with Calvilnguy, that's it'd be better to just write cross-platform JS scripts.