Same problem.
I have 3 WP blogs.
The two I started from scratch with WP 2.x are OK.
But the one I upgraded from 1.5x to 2.x does not let me preview.
No work around I could find aside from publishing, editing, then changing the post timestamp.
Until we solve this glitch, I turned on OPTIONS > WRITING > Users should use the visual rich editor by default.
This way I can see my graphics —?though the visual rich editor causes other problems.