Well the clock rates for SB are about the same for the first batch of chips and the IPC is better on SB. They also appear to have a lot of headroom for overclocking so I imagine that as time drags on, they'll have offerings at a higher stock clock rates.
I think Intel held back on their first run on parts so that if they were having problems with the new process for IB, they could always release newer parts with higher a higher clock until they can get IB ready. Might also have to do with having maneuverability to deal with whatever AMD comes out with in the next several months.