As RJR said, you can overclock the multipliers on the K series chips for Sandy Bridge coming in DEC/JAN
Clock rates are determined by a crystal which sets a frequency, a series of logic chips can multiply and change that frequency output. Currently, there are several crystals and even more logic in chips in order to accomplish the various frequencies needed. Intel is trying to only have one base crystal and a variety of multipliers to get the buss speeds for the CPU, RAM, PCIE BUS, SATA BUS, etc. As more and more of the "chipset" moves on the the CPU, this simplifies their design.
You will still be able to adjust the multipliers for Memory, and (for a K series chip) CPU.
The problem is that your HDD will go haywire when you overclock it (since everything will be connected, when you OC your CPU via the base frequiency, your USB, HDD, etc will also overclock). It's possible motherboard makers will find some workaround, and it's possible that the Socket 2011 Chips won't have this limitation. We'll all find out all the details over the next month or so.
I find it hard to believe no motherboard manufacturer will add a prescaler+PLL set to the USB/SATA/PCI-E controllers. It should be painfully easy to do and would make overclocking so much easier (or no harder than it used to be).
As for why they locked the multipliers, actually, they kinda always did that, only unlocking the 1000+ dollar extreme editions. They now unlock a few of the cheaper ones, marked as "K" versions.
If what you meant by locking is why is Intel tying everything to a single clock, then I can think of a few reasons. It's cheaper to design mobos, reduces a few complications and possible instability from bad sync., and also prevents internal competition from the lower end chips.