From some discussions around the web, it looks like Google might be altering the code in its results pages (SERP – search engine results page). Whether this is to deliberately stop rank checking tools (like Aaron Wall’s Rank Checker plug-in for Firefox) isn’t clear – it might simply a be a side-effect of some changes Google are making.
Hopefully the developers of these tools will release updates soon, but if you use one of these tools and notice problems, it could be down to these changes.