It's pretty odd. I thought maybe it was when a post was published or something that the htaccess would be rewritten, but I tested that and it's not it.

I've compared 2 blogs I have, running the same version of WordPress, one it happens on and one it doesn't, so my suspicions fall to plugins...

I'm wondering about the Filosofo Home-Page Control as it's the only plugin I use on the problem site that involves modifying the htaccess file...

It doesn't make sense though...

plus, you don't use that plugin do you Richard?