Hummmm... OK, well it seems I am not *quite* the moron I thought I was :S
I think I may have found a bug, but I'm not sure exactly with 'what'. I have now recompiled the very same kernel about 10 times or more in the last 24 hours.. At the last going-off, here's what I have seemingly determined (and yes I need further testing to be 100% certain) but what I AM certain of now, is that I did not neglect to load the config file (er, well maybe I did once or twice, but that is not the problem).

With two otherwise identical kernels:
Kernel A - uses deadline IO scheduler and voluntary preemption.
Kernel B - uses Anticipatory IO scheduler and full preemption (low-latency desktop)

The kernels are otherwise 100% identical. The modem driver will compile, install and modprobe perfectly on Kernel A. It will NOT modprobe on kernel B.

I took Kernel A, compiled it fresh, rebooted, put the video driver in, got on the desktop, and built and installed the modem driver. Modprobe worked perfectly.
Then I went to the kernel source, did make clean, make mrproper, make xconfig, loaded the SAME configuration file I saved from building Kernel A, changed the IO scheduler and Kernel preemption, saved, made, made modules, LILO, reboot, install video driver, go to desktop, build modem driver, install, WON'T modprobe - Invalid Module Format.

Besides the obvious, of changing only ONE of these 2 things and going thru this again, I'd appreciate any recomendations as to what if anything I should do before becoming certain this IS a bug somewhere? I'd like to prove it one way or the other.

Just try re-compiling/re-installing those modules. Probably there are some undesirable effects due to the elimination of the previous I/O scheduler (there are a lot of urban-myths around this but that should probably solve your problem).

If the problem persists, leave the .config alone and do the exchange of schedulers through Grub or Lilo.

Of course, I assume you are sure that you don't had misplaced your fingers. Good Luck.