There's been a bit of a buzz lately on ATP testing, to measure cleaning levels. ATP measures biological material present on a 
surface; hence it's a good measure of cleaning (have you removed all the "stuff"?). However, some folks confuse cleaning with sanitation or disinfection. If you've disinfected a surface, but not removed the dead critters, you'll show contamination, because the bio markers are still present. Secondly, it seems that ATP meters are thrown off by some disinfectants.
Go figure.
Here's a study from National Institutes of Health site, summarizing the issue. An excerpt:
Results
All four ATP meters tested herein demonstrated acceptable linearity and repeatability in their readings. However, there were significant differences in their sensitivity to detect the levels of viable microorganisms on experimentally contaminated surfaces. Further, most disinfectant chemistries tested here quenched the ATP readings variably in different ATP meters evaluated.
Conclusions
Apart from their limited sensitivity in detecting low levels of microbial contamination, the ATP meters tested were also prone to interference by different disinfectant chemistries.
