Yeah, I run 1Thz, but only on my Netburst CPU, my Xeon in my sig can't OC quite that high....
There may have been some in the low 8Ghz range, but I think mostly they stopped around 7Ghz at the very top for the CPUs we use. Sure, there have been 30Ghz chips, and 300Ghz chips, etc... But they're not anything like we use for our computers. They're for other purposes. It just means the transistor is switching very fast, not that it's actually doing anything worthwhile.
Me personally.. The most serious overclocking I did was on my ancient 3.0C back in the day. IC7-Max3 with an SP94 heatsink. 250x15=3.75Ghz, fairly benchmark stable. I think it had a slight problem here and ther at that speed, but 3.6Ghz was fine, very hot room too(still living at home then). I was able to post at 4Ghz on it, which of course means nothing, but that is still worth mentioning since I was on air. I took an almost stock Radeon 9800 Pro and hit 22,000 3DMarks in 01 with that system, beat my friend with a 2.6Ghz A64 Venice at the time! I liked that system. Now, I sit at stock.
Oh, and the post above me for the 8Ghz "P D" isn't a Pentium-D, it's a P4 631. It's the 65nm P4, very high clocking. Only thing that could typically clock higher was the Celeron version.