Core 2 has definitely turned a lot of heads and changed a lot of minds. We were expecting a comeback from Intel after the X2 trounced the dual core Pentiums again and again, and Core 2 delivered. It shows even more promise, which the enthusiast community is helping to demonstrate. Recently, the Core 2 Extreme was taken to an amazing 5.7GHz using LNG cooling, nearly double the processors stock clock speed:

The processor, was of course, a X6800 while the motherboard was an ASUS P5W64. Using liquid nitrogen OPB managed to push his X6800 processor to over 5.7GHz from the stock frequency of 2.93GHz, close to a 100% overclock.
While exotic cooling solutions such as this aren't even close to the norm, it shows that the Core 2 architecture has a lot of headroom to continue to grow, especially as Intel pushes towards the 45nm process. It won't surprise me in the least to see Intel slowly nudge the Core series towards 4GHz and beyond in the near future, after quad-core CPUs make their debut.