The calendar accessible by clicking on the clock applet from gnome-base/gnome-panel always displays the first day of the week as Sunday, regardless of locale. (Most of the world has the first day of the week as Monday.) There's a patch on the above URL that fixes this; however it's not going into Gnome 'cos of feature freeze and the fact that the calendar control will be fixed in gtk+-2.4 (which stable users won't be seeing till Gnome 2.6); it'd be nice to have this in the Gentoo ebuild; regardless, I'm posting this bug anyway just to get it in the open. I'll put up a patch that applies without fuzz (probably with some offset) to gnome-panel-2.4.2; I'm not fussed if you choose not to apply and have to maintain it.
Created attachment 24060 [details, diff] Patch to effect fix
patch looks clean enough to me to put in for the time being.
Thanks for the patch, good work. I've committed it now to gnome-panel-2.4.2.