Ok, let me get this straight as I'm about to build a sandy bridge for a client.
1) Why did Intel release Sandy Bridge with two different stepping (D1, D2) on launch?
2) Why couldn't Intel release all Sandy Bridges with D1 stepping in the first place?
3) Last but not least, the chips with the "K" moniker have unlocked multipliers and the ones without the K are not overclockable? Did I get it right?
You can do some minor overclocking with non K chips, but it won't be much. The K chips unlock the multiplier, which is where the vast majority of overclocking will happen with SB. Any changes to the base clock with create system wide instability since everything runs off that base clock. I have seen people up it from 100 to around 103, and some claiming about 105. Yet that is not much overall (105 x multiplier vs. 100 x multiplier). You can also do a slight bump within turbo. Yet if you want to really overclock, you want a K chip.