Having
/var/log/Xorg.0.log end up being 0 bytes tends to happen if the system panics/reboots before the buffer is flushed to disk.
If you own any other computers you could ssh to this system and
tail -f the log, and you'll have a copy of the log up until the point of error.
I'm not sure if
EXA/XAA is acceleration still supported by OpenBSD's intel driver, but you may wish to try them.. as well as a snapshot to confirm the issue hasn't been already been fixed.
After that, you may want to report the bug to the developers.
http://www.openbsd.org/report.html
http://www.openbsd.org/mail.html