Solaris 10's xorg setup has the nasty 'ability' to use the resolution it wants (it doesn't matter which resolution you've chosen). To prevent this behaviour, add the option 'Virtual' in the corresponding 'Display' subsection in /etc/X11/xorg.conf.

For example, to force 1440x900 resolution use this:

Subsection "Display"

        Depth       24
        Modes       "1440x900"
        ViewPort    0 0
        Virtual     1440 900