Neo, very nice hack. Very very cool!
Simple but great idea.
I implemented it with a couple of enhancements. Just to give you an extend-idea...

- stored a little bit more infos coming from user and thread (for example: forumid, number of posts in thread that moment, proxyip, etc).
- I already have a weekly scheduled "dump, archive and flush" of all four (now five) log tables. So I removed the in-code delete timeout.
- To make that table not too fast grown up, I added a field to forum tables (managed in forum.php) to choose either if log accesses to that particular forum or not.
This way, I could focus the logging to most 'sensitive' areas of my board.
If someone needed, I could post the delta instructions.
Any way, many thanks to your idea and clear implementation.
Bye