See attached image.
I'm running dual-head on a SONY VAIO laptop (VGN-NR298D), with another 19' LDC; Dual-head seems working except that 10% of the right-most screen of the 19' LCD is not "graphically used": Any windows can be placed there but it won't trigger the redraw of the part of desktop, neither would the wallpaper fill that part. See the rightmost of the attached image, the "windows" don't actually exist. They are the ghosts of the ones that were there couple minutes ago.

I've never had the problem on Dapper.