I had a page ranking for a keyword that is nowhere on my page, nor in any backlinks... it was on 7th spot for a few days (way in-front of any other keywords which are between 20-40th place at the moment).
I know google searches for synonyms, but this was far off of what i'd expect. Also for some things it is difficult (almost unnatural) to write a text with only 2.5% density and i hope google knows it.
Considering the algorithms we can make i don't think google would need to rely on keyword density as we can determine the quality of text in better (still mathematical) ways. Still, i abide the 2.5% rule for now.