We have been covering the issues with search ranking checkers since beginning of this month. At first, we thought it might be related to WebPosition Gold only, but it was not, it impacted most, if not all, scraper based rank checking tools, checking Google's search results. A week ago, WebPosition Gold hinted that a fix is coming to their software, despite that Google hates rank checkers. But no fix has come yet, as far as I know.
To make things worse for rank checking products that scrape Google, it seems like Google is dynamically messing around their HTML structure to confuse the scrapers.
The continued discussion on this topic at WebmasterWorld shows signs of this. One member, pageoneresults, said:
Ah-ha, they've got some sort of dynamically generated thing going on with the output. Every 24 hours it is scheduled to generate a variation in the html output. Google just put it on auto-pilot after testing these past few weeks. < Me "Tin Hat" is always on during these types of discussions. :)
After that post, member sc0ttkclark said the format changed once again.
It seems like Google may have implemented a feature that is transparent to searchers but can totally mess up rank checking tools. This is not just impacted WebPosition Gold, but all scraper based tools, including the SEO Firefox plugin, which is being discussed at Cre8asite Forums.
Update: Google's JohnMu directed me to a Google Blog post named Search experiments, large and small, where he said, "I imagine it might be more due to" that blog post. I was away when Google posted that, but as John said, it does look to be very related.