For what it is worth, I've been able to make the mirroring happen on my laptop as well. I usually use the proprietary driver for the nvidea GO 7300, but I messed around with a couple of different setting until I finally got www.teaspring.com
to mirror for me.
What finally worked was a combination of the nv driver and firefox 3.5.5.
That's it, don't really have a solution other that switching back to the proprietary driver or maybe vesa or fbdev.