All changes in relation to service tax take effect from a notified date and not immediately as in case of excise duty.
The notified date being 1st April 2012 in case of changes in relation of service tax . Hence Service Tax Rate will be 12% from 1st april 21012.
And changes in rate of excise duty take effect from midnight of 16th March 2012.
//indiabudget.nic.in/ub2012-13/cen/dojstru2.pdf
See Point No 1 & 5 of Part A of the link