Southern Islands isn't concrete. I've heard that AMD wants to get it out this year but it appears that more effort is being placed on Fusion parts. They inched into Q4 2010 from 2011 in just the past few weeks. There is also handful of new sockets coming from AMD as well. It's a bigger change than AM2+ to AM3.
Where have you heard that? Everything I've read strongly indicates that SI is right on track for a September/October release.