I went through the same problem. as of 2.5.48, the old modutils don't work,
and you need to grab the new stuff from the link below.
when you install them, they will rename the old insmod/rmmod/modprobe to
*.old.
if you drop down to 2.4 again, then the new utils will simply call the old
ones, so it is pretty transparent.
I really don't know where it's written about this change, but I went through
a lot of pain to find this out. perhaps I just missed the obvious in some
readme file...
http://www.kernel.org/pub/linux/kernel/people/rusty/modules/