Minecraft will be getting dual-core support in august! I for one love this, because Java is usually god-awful slow.
I never noticed any lag on my pc. Not on my hexacore and not on my dual core, and my dual core ran on 1.9GHz, so it doesn't really help for me.
When it's in the mood, my relatively sedate laptop will run it on max graphics at about 24fps and no lag It is dual core, though, so I suppose multicore support could help..
It's really useful to know that you tell everyone you don't play Minecraft on a topic that's about the fact that Minecraft is getting multicore support... not.