I have the same problem you have. My workaround is to start X without a window manager after boot, exit X with control/alt/backspace, and then start X with window manager:
[watch the stippled screen for a few seconds, hit cntl/alt/backspace]
Depending on how you setup X, you may need to to perform other magic to start up X without window manager.
I have absolutely no idea why this works, but it works for me.
The problem started when I upgraded to FreeBSD 4.8. Downgrading X to 4.2 (from the 4.7 CD) didn't fix the problem, so it seems to be a bad interaction between FreeBSD 4.8 and the nvidia driver.