For those of you noticing some lagging, often when doing background syncs, this is not really down to the processor - but the OS.
Every Android device I've reviewed has lagged or stuttered at times, with the exception of the Galaxy Nexus - running ICS. It seems Android OS may have finally fixed a number of issues, so when the Xperia S gets ICS - any UI issues should disappear. ICS, for example, supports hardware acceleration.
So, it's probably fair to say that the Xperia S will get even better when it gets ICS - not just a new look.
I'm struggling to imagine why I'd need a quad-core processor, as the Galaxy Nexus was awesome and I loved it, except the camera. The Xperia S has a great camera, and the dedicated camera button (plus the quick start mode) so it ticks all the boxes. I really don't care about the next device to be announced for the summer.
For a tablet/laptop, I can see where quad-core may work - especially if it's bigger and gets a bigger battery too.