In the days when the 333MHz Celeron A was the fastest Celeron processor out, power hungry users still had a reason to purchase a Pentium II, as it was offered in clock speeds in excess of 400MHz, and for those that didnt overclock, that was a considerable difference this was Intels marketing plan for the caste system among their processors: the Celeron would be limited by an absurdly low clock speed (comparatively speaking), and the Pentium II would lead the pack with a much higher (and more expensive) offering. For those that overclocked, the 300A running at 450MHz was already faster than the fastest Pentium II, making the Celeron Intels one "undiscovered" hit wonder.
As soon as AMD began to catch up in clock speed, Intel gave the go-ahead, and announced the resignation of the Slot-1 300A and the entry of the PPGA (Socket-370) 300, 366, and 400 processors, with the latter two making their way into the slot-1 market as well. In actuality, the only change Intel probably had to make to get the 366 and 400MHz parts out the doors of their fab plants was resetting the clock locks on the processors to support the 5.5x and 6.0x clock multipliers necessary for the two processors. From a marketing standpoint, the introduction of a 366MHz+ Celeron would mean more intense competition for AMD at the low end of the price spectrum of the market; and if the Celeron would attempt to interfere with the Pentium IIs sales, the response any salesman at a retail chain would give a questioning buyer stuck in the Celeron 400 vs P2 400 debate would be, "the Pentium II 400 uses the 100MHz bus, which is a full 50% faster than the 66MHz bus the Celeron uses." Too bad most people that dont know that the real performance difference is negligible will let that statement ride and spend the extra cash on the Pentium II 400 system. This is why the Celeron 400 runs at 66MHz x 6.0 while the Pentium II 400 runs at 100MHz x 4.0, purely marketing reasons.
As far as official specifications are concerned, the Celeron 400A is clocked at a 66MHz FSB frequency (as referenced above), using a 6.0x clock multiplier, at a core voltage setting of 2.0v and does not feature the embedded serial id number the Pentium III "boasts" having.
How did Intel make sure that a Celeron always ran at a 66MHz Front Side Bus (FSB) frequency and a Pentium II always ran at a 100MHz FSB? The easiest way possible, making the individual chips locked to a certain multiplier, making operation at any FSB other than its rated frequency, extremely difficult if not impossible; this is Intels most popular combating force towards remarking (selling processors rated at lower speeds as if they were higher speeds and charging more for them, i.e. selling a Pentium II 300 as a Pentium II 450 for twice as much) known as clock-locking. You will notice that "clock-locking" is never referred to as an overclocking deterrent, this is because, Intel never truly had any objections to users overclocking their chips, it is just those that insisted on making an unlawful profit on such experiments (i.e. remarkers) that would give Intel a bad name and therefore forced them to pursue this avenue of escape. If a consumer spent a horrendous amount of money on a Pentium II 450, and it turned out that the processor was nothing more than a 300MHz Pentium II remarked as a 450, and it was sold to them by a fly-by-night company the only group left to blame would be Intel - this was something Intel would definitely not stand for.
The other type of processor "locking" that has received its fair share of media coverage has been "bus" or frequency locking which, in theory, would involve a modification to the processor to allow it to run within only a specified range of clock speeds derived directly from the FSB setting it was run at. The humorous part of the frequency locking discussions we've all been having for quite some time now is the fact that Intel never publicly admitted to pursuing such an avenue with their processors, although Intel will continue to implement restraints on remarking with each modified processor they kick out of their fabrication plants, frequency locking is one that has never been admitted to. Will future Intel processors be frequency locked? They may be, however for now, as far as the current generation of Intel processors are concerned (including the Slot-1 Celeron 400A), there is no implementation of frequency locking in practice by Intel. The Pentium III was originally stated as being the first frequency locked processor from Intel, however with its release less than a month away, and still no official word from Intel, the last week of February should be an interesting one. Intel's current samples of the Pentium III are shipping without any frequency restraints, and a quick search among on-line vendors will reveal that OEM Pentium III processors are in fact, available and in stock, although the quality and reliability of the vendors that offer them is up to your own experimentation, at your own risk. Frequency locking may become a necessary evil in the future, but for now, you're safe, even with the Celeron 400.