It seems broken starting with CS5.5.
(I'm on Windows).
The problem seems resulting from current directory (folder) not being set when you open a script or FLA. So, relative paths still work but they are always relative to the root of C drive (in our tests) which of course defeats its purpose.
You can use the URI class from XJSFL to convert your relative path to an absolute path but you have to download the XJSFL framework and initialise it in your JSFL file in order for it to work. You can download and find out more their website www.xjsfl.com