slusb module can be compiled for amd64 too.
Created attachment 144410 [details] slmodem-2.9.11-amd64.patch from http://linmodems.technion.ac.il/packages/smartlink/slusb_x86_only.patch.tar.gz
Created attachment 144412 [details, diff] diff for slmodem-2.9.11_pre20080126-r1.ebuild
Could you poke upstream about integrating it? This is a PITA to maintain on any updates.
Fixed in slmodem-2.9.11-20080401, thanks!