I've had to give up on getting a serial modem to work with Fedora 6. The best tip I ran into on this was here:
I had installed the xen kernel, just to play with it. It seems the xen edition makes the serial ports unavailable to the O/S. That makes some sense. So I tried installing Fedora without the virtualization option (since installing it with virtualization fails to install the non-xen kernel as a boot option).
However, the non-xen install gets to the point of actually formating and copying over the selected software and crashes. I tried it five times with simpler and simpler options selected. Everytime, the install just crashed.
This is the 64-bit Fedora 6, on a brand new AMD dual core, a mid-range motherboard, no extra video card or anything else fancy, a couple of big SATA drives (also new) and 2 GBs of memory.
Linux normally works really, really well these days. But Fedora is pathetic.