The workspace switcher on screen :1 in my dual-head setup shows the switcher for screen :0. The switcher will be attached to the correct screen if removed from the panel and created anew. It will again be attached to screen :0 on the next Gnome restart unless it is configured to display the workspaces in 1 row. (N.B.: As far as I remember, the switcher at :1 was attached to :0 although it showed the workspaces in 1 row with the default settings, so I had to remove and add the workspace switcher at the panel in :1 once to get it working.)

Steps to reproduce:
1. Create a dual-head-setup
2. Log in to Gnome and open a window on screen :0. The new window appears on the workspace switcher on screen :1. If not, set rows in switcher at :1 to 2 and relogin.

I'm experiencing this with the dual-head setup for my ATI fglrx with aticonfig (sudo aticonfig --initial=dual-head --enable-monitor=lvds,tmds1 --screen-layout=right), Ubuntu 8.04, Metacity 2.22.0 (no compiz), Workspace Switcher 2.22.2, libwnck 2.22.3.

Bug #480710 could be related, but the original poster doesn't say anything about a dual-head setup. I guess bug #558544 has the same cause as described here but also addresses issues in bug #441658 and bug #552744.