Seems not to be in portage, but belongs to xfce-goodies.
<snip> This plugin checks for package updates using the Smart Package Manager. Works with both Fedora (RPM) and Ubuntu (DEB), tested with F-8 and gutsy. It can also be configured, though with somewhat less features and functionality, to use either the Yum (+Extender) or APT (+Synaptic) package managers. </snip> Fail to see what's this good for w/ Gentoo.
You are right, didn't see that ;) Thanks for Info!