As you say, extreme cooling. The Pentium Netburst based CPU's (particularly from the Prescott and later) could generally hit high frequencies but were only really limited by their rediculous heat outputs. Intel were slowly getting this under control and by the time that they released their refreshed 90nm 2mb L2 Cache Pentium 4 6xx CPU's, heat wasn't so bad and around 4GHz on air was usually the norm. The last of the Pentium 4's were the 65nm die shrunk Pentium 4 6x1's, which were pretty mad overclockers. They didn't get much hype as the Athlon 64's were pretty much blowing them out of the water, but it wasn't rare to see the 65nm Pentium 4's hit in excess of 4.5GHz, some which even made it as far as 5GHz on air cooling.
Don't forget however that clockspeed isn't everything and even at 8.1GHz, such a Pentium 4 wouldn't be anything special at all!