I'm using PHP5 under Debian 5.0 and I've upgraded WP within a week for each new version coming out. I don't have Twitter tools, and the commenting out of auto-save doesn't work.
As a workaround, for the past six months I've been writing my posts in a text editor, then pasting it into a blank post in WP, saving the draft, getting the error, going back to my list of posts in my Admin screen, clicking "edit" to get back to the edit screen, then pasting in the post again (unless it auto-saved before I hit save). It'll usually work OK after that.
I would try disabling plugins, and reverting to default themes, but I think that's a bit pointless since I have plugins and themes for a reason. I dunno - it's an incredibly annoying bug, but I'm willing to wait it out for however many years it takes for it to get fixed. I've been living with it almost daily for over a year now, so I guess I can last a few years longer...