i had similar problems with xserver 1.5, nvidia, xinerama, 3 monitors and rotation.
Console switching wouldn't work (Got an empty console window, typing commands worked though).
Furthermore there where some nasty graphical bugs which only occured with rotation.
My solution was to switch back to xserver 1.4.
If you a recent ports tree you can do this with a supfile like this:
*default release=cvs date=2009.01.23.00.00.00
*default delete use-rel-suffix
For me it even worked to build x11/xorg (7.4) without any problems:
$ pkg_info -I xorg-7.4 xorg-server-1.4.2,1
xorg-7.4 X.Org complete distribution metaport
xorg-server-1.4.2,1 X.Org X server and related programs