Why aren't there 1.7GHz Pentium 4's available? Because the design isn't ready. If it was ready, it would be shipping. Just because someone can overclock a part to 1.7GHz doesn't mean that that same part could be marked as a 1.7GHz CPU and sold in the retail channel.
First off, there could be some bit of code that this overclocker hasn't stumbled across yet that will crash his/her system. I remember when Unreal was first released a whole bunch of "stable" OC'd systems crashed all over the place. They'd phone Epic, Epic would say slow down the clock, and everything would be fine. There have been others. Recently a whole bunch of people complained that 3DMark2001 crashed their stable systems. In any case, customers who paid money for a processor don't want it crashing on random bits of code, and generally get upset and demand recalls when such things occur.
Second, OC'ers are very careful on heat. They use well ventilated cases, they put on big heatsinks, they monitor their temps. This isn't practical in the real world where systems need to be used in very hot climates.
Third, OC'ers often push up the voltage. They aren't concerned that the little bump that they just did shaved a sizeable percentage off the life of the part, they want the speed now and they think it will last long enough for them anyway. Corporate customers want their machines to work for the advertised amount of time. So there is a limit to how high a manufactuter can set the voltage and keep reliability high.
And the last one that I can think of right now is that chips slow down with time. It may run at 1.7GHz today, and probably tomorrow, but in two or three years, it may not, and in 5 years probably not. A company needs to mark a product at a rated frequency and a customer can rely that it will work at that speed even over a long period of time.
There are probably more reasons.
I can assure you that when 1.7GHz Pentium 4's are ready, they will ship. This is the way the industry works.
And the comment that AMD and Intel are working together to slow down progress - this is almost laughable. If they did, then some startup would show up out of nowhere with a new wonder-product, have TSMC and IBM fab it for them, and they'd make a fortune. I can assure you that Transmeta did not originally intend Crusoe to be a mobile/laptop part, they wanted something that would compete in the desktop space, but they weren't fast enough and are now in the mobile arena. There are other Transmetas out there. Even so, if AMD and Intel somehow came together to slow down progress, Transmeta and Via would keep moving forward and would improve their products and their products would eat market share.