This issue stymied me for quite a long time.  There are other reasons, and hence other solutions, as to why you’re getting this error (couldn’t hurt to double check file permissions), but for me, the solution turned out being that I needed to dump /var/sessions from my Magento installation.

I’ll update this post if the solution doesn’t last, but Magento and my other PHP apps have been running without a hitch for an hour now.  If you’re using Ubuntu, also try an apt-get update, apt-get upgrade.  I think this all started because a bug may have been introduced somewhere along the line during a previous upgrade (mySQL maybe?).

I’m just glad to put this behind me… let me know if this works/doesn’t work for you.