IINM, the thickness of Pentium D & C2D processor is just the same, maybe it's just you're imagination...

Well yes, the lock is hard, why?

Because intel wanted the heatsink to be lock tight so there will be zero space between the processor IHS and the Heatsink base...

They did that as to achieve good contact between the heatsink base and the processor IHS...
If the contact of the IHS & the heatsink base is not good aka not tight, then the heat transfer will slow down @ doesn't work at all...

**IHS - Integrated Heatspreader - The metal part on top of you're processor

I hope this explains

Yes...
Wait until you're using 3rd party heatsink such as Ultra 120 that uses bolted in lock type, now u see the stock intel heatsink is bending you're motherboard, then later after you change to 3rd party heatsink, the bend will go further more...

But just remember not to lock it to tight or it will kill you're mobo...

Lock it enough, but not too tight...
This is just an advise before you change into 3rd party heatsink..

All heatsink will surely bend motherboard one, except for those that doesn't need something to bolted through the motherboard, even the stock intel LGA775 heatsink is using bolted in lock style, so practically, it's normal, just remember to look at the bent, don't let the bengkok so much, or it will kill you're mobo.

If the bengkok is just in small percentage, then there's nothing that you should worry about...