As a workaround, you could make a startup script that deletes the file and makes a symbolic link to /dev/null, effectively discarding everything written to it.
Code:
#!/bin/bash
rm ~/.xsession-errors
ln -s /dev/null ~/.xsession-errors
Obviously, by doing this you opt out of any interesting tidbits that may be output to there. I found that the only error logs I really ended up reading were /var/log/Xorg.0.log{.old} and /var/log/kdm.log anyway.