inkscape-0.45.1-r1 ebuild has added back the jabber useflag. However, it incorrectly depends on >=net-libs/loudmouth-1.0 when this useflag is enabled. As I know, earlier versions of inkscape depended on loudmouth, but current versions don't depend anymore, they've implemented a jabber client by themselves (I guess it was done during Google Summer of Code). My system does not have net-libs/loudmouth installed and I have a modified inkscape ebuild with jabber support but without that dependency, and inkscape builds fine.
I think you're right, configure doesn't check for the lib and works w/o it. Dep removed, thanks for the report.