I searched google for a fix to getting slmodem to compile under kernel 2.6.19 and could only find a couple crude fixes
one was a .patch file that i personally could not get to work
After stairing at the patch file and the other things i ran accross on the net i came up with a fix
this was done with slmodem-2.9.11-20061021
after unpacking cd into slmodem-2.9.11-20061021/drivers
edit amrmo_init.c
remove this line:
#include <linux/config.h>
save and exit
cd .. back to slmodem-2.9.11-20061021
make like normal

this fixes the errors that look like this
/path/to/slmodem-2.9.11-20061021/drivers/amrmo_init.c:589: warning: passing arg 2 of `request_irq' from incompatible pointer type
make[4]: *** [/path/to/slmodem-2.9.11-20061021/drivers/amrmo_init.o] Error 1

hope this saves someone else a headache after upgrading to the new kernel.