AFAIK Atom is a rebranded Celeron usins a new fabrication technique and core optimisation to reduce power usage... Thus gives upto 1.5x times more performance over conventional single core celerons @ same clock... I read it somewhere in Wikipedia.. Maybe under the article describing Intel Atom??
For more on Atom, see-
*www.tomshardware.com/reviews/intel-atom-cpu,1947.html