The Open MapQuest tiles are rendered by the company MapQuest who are a subsidary of AOL. Since about a year they have increasingly started to use OpenStreetMap data and have brought a variety of different services out based on OpenStreetMap. The open tiles are one of those offerings ( http://devblog.mapquest.com/2010/07/09/mapquest-opens-up-uk/ ).
The tiles are (as far as I know) rendered with the same technology (Mapnik) as the osm.org map, but uses a different style sheet, that was designed by MapQuest to look similar to their commercial map style. The style has been published and is available under an open license.
The tiles are typically updated every 15 minutes from the latest OSM data and they should have sufficient server capacity.
More information about it can probably be found in some of the blog posts on the MapQuest developer blog
answered 15 Apr '11, 05:20