I noticed a peculiar behaviour that the plugin Simple Tags has.
When the plugin is activated in version 3.1. the is_home() function always returns true (or 1 obviously). this only happened after I upgraded so I am certain the upgrade was the reason. All other plugins, I have installed do not interfere in this way.
I switched it off for now, but I was wondering how I can report this issue. I am brand new to wordpress so I may need some guidance or if someone feels friendly, maybe do it for me (it's worth a try :D)