If you have access to the FCTM (sorry Boeing Flight Crew Training Manual) you will find a section called abbreviations at the front. If you look under ATM sure enough it defines ATM as assumed temperature method.
Hulshof's Performance reference handbook for the 737 does not agree with Despegue's suggestion of increased V1 speeds when operating on contaminated (note the difference from wet) runways. To quote: 'since the reduced stopping capacity is more significant than the reduced acceleration capability, V1 needs to be reduced to a lower value.'
Common sense and experience with events like Air Berlin at Dortmund suggest that runway friction measurement is also an inexact science. Reducing margins on contaminated runways is not a good idea when you do not really know how much you have in hand.
Judging by his chosen name Santa737 probably operates somewhere more often contaminated than I do. On the relatively rare occasions I cannot use assumed temperature I find the restriction no big deal. In fact I like it.