For the record my version 1.5.9 returned
DB function failed with error number 1016
Can't open file: 'jos_session.MYI' (errno: 145) SQL=INSERT INTO `jos_session` ( `session_id`,`time`,`username`,`gid`,`guest`,`client_id` ) VALUES ( '651f6d2e67125ec8f91a968389c88a9c','1238364874','','0','1','1' )
Panic. A quick search with Google led me to this page (thanks everyone who logged this problem and passed on the temporary fix). I could not do a normal REPAIR as the jos_session table was completely missing. I picked up an old session table from a backup using SQLAdministrator and restored it to the site. That kickstarted my joomla site and all seems to be functioning well again.
But why did the table go missing? It could have been my ISP using the depths of a Sunday night to do some kind of maintenance but I doubt it. The fact that so many Joomla people from various sites seem to have hit this problem all at the same time points to something else.
A new version of the software, the 1.5.10 security release, came out yesterday and has 66 bug fixes. I see no mention of the jos_session table problem but I am hoping that it may have been attacked. After all, if you put "jtablesession" into the forum search 10 pages of threads appear of people having this issue. This thread alone goes back to midway through last year.
Meanwhile, I would encourage administrators to keep reporting. Perhaps the underlying cause will become apparent.