My use flags in /etc/make.conf contain +svg, however KDE (3.5.8) applications (namely konqueror) doesn't support SVG. This is caused by missing kde-base/ksvg which provides required kpart. I have no experience with KDE, but I think use svg trigger should be placed somewhere. Note I have no kde-base nor kde-meta installed (I want only some KDE applications, I'm not interrested in the whole desktop enviroment).
If you want svg support you should certainly install the package that provides it if you choose not to use the whole thing anyway. We could theoretically add a USE flag that would simply add a dependency but we don't do that because it's rather pointless.