Hi,
I just installed FC6 x86_64. In the past I used yumex --forcei386 to access the i386 packages (mplayer,mplayerplug-in,firefox, and all their deps). Unfortunately the developer took that option out. He though that people would be too dumb and screw things up. That was the only good thing about yumex. In any case, does this mean that I have to make .repo files explicitly for i386?

Should not have to but you should specify that you are wanting to install the 32-bit package(s). I personally prefer and use kyum, but in cases like mplayer and all the required dependencies it is better to download them to one location with the 64-bit counterparts for the libs and install them all at once.

The Fedora repos already have the 32-bit and 64-bit applications and libs available in the x86_64 directory, some of the third party repos do also, but not all, so yes in some cases you need to create a special repo file explicit for 32-bit needs in these cases.