Abstracting with credit is permitted. To copy otherwise, to republish, to post on servers, to redistribute to lists, or to use any component of this work in other works requires prior specific permission and/or a fee. Permissions may be requested from Publications Dept., ACM, Inc., 2 Penn Plaza, Suite 701, New York, NY 10121-0701 USA, fax +1 (212) 869-0481, or permissions@acm.org. C 2008 ACM 1550-4832/2008/08-ART12 $5.00. DOI 10.1145/1389089.1389092 http://doi.acm.org/ 10.1145/1389089.1389092 ACM Journal on Emerging Technologies in Computing Systems, Vol. 4, No. 3, Article 12, Pub. date: August 2008.