/etc/make.conf should list the wacom under the input devices or anywhere there.
Then "emerge -s wacom" as likely there has to be some form of software that is needed to talk to the wacom similar to ndiswrapper + likely some input key bindings.
I would first try this on say 3.4 mini or 3.4e /f as then you should have a better idea about if it works + likely something is missing from 3.5 beta that we can add later.
If all else fails try pclinuxos latest version and if working in there then it should with a bit of effort work in SL + you should be able to copy or clone some settings from pclinux over to SL like /etc/X11/xorg.conf with the input part / mouse part or similar.
tucows / other linux download sites might have alternative drivers for the wacom or generic source code ones you might be bale to use but I am sure by now there has to be a easy way / software to do this / get it working. http://www.google.co.za/search?q=wacom+ ... =firefox-a http://linuxwacom.sourceforge.net/