I have searched but haven't come across this.

Last night on BBC South Today they featured a piece on a trial taking place at LGW - http://news.bbc.co.uk/1/hi/business/6203636.stm

Putting the enviromental impact aside I've always wondered what the economics of doing this are. On a typical push to take-off an aircraft will burn X amount of fuel at a cost of Y. If the cost of the tow (assest, staff, etc) was at or near Y then it should be a no-brainer. Not only will their be a reduction in air pollution but there will also be a reduction in noise polution.