A good or bad thing?
One of the big questions Intel must ask is whether or not the existence of the Celeron is actually a good thing. It's a low-margin CPU meant to compete with the lower-speed AMD/Cyrix/IDT CPUs, but is based on a similar core to the high-end Pentium II processors, and so is capable of high-end performance if so-clocked.
In a slot-1 situation, the Celeron is a boon for savvy computer users. Anyone could purchase BX motherboards and PC100 RAM, and a Celeron 300A, either to run as-is or overclock. Later, it would be a simple matter to swap out the CPU for a P2-450 (assuming you didn't overclock ) or even a Katmai when it's released later in the year.
However, Socket-370 presents a few problems for this upgrade scheme. For one, it hits a brick wall. There are no processors besides Celerons which will take advantage of the 370, and as a rule, Celerons will never surpass their bigger Pentium II brothers (unless overclocked). Buying a Socket-370 motherboard basically locks you into the Celeron line for as long as Intel decides to support the Socket-370. This makes the PPGA Celeron much less enticing than its Slot-1 twin. The cost savings from purchasing the socketed chip along with a more or less proprietary motherboard may not be worth the trouble to most people.
The Socket-370 Supermicro 370SBA
Excluding the system case, the motherboard is the most troublesome component to replace. Every peripheral and on-board component must be removed, and once set back up, there's no guarantee Windows will gracefully detect all of your new system devices. Luckily, there may be hope. Several companies are working on socket-370 to slot-1 converters (basically taking the chip and putting it back onto an SEPP package), that should allow you to save money on the actual processor and fit it into your current or new BX/GX motherboard. Whether or not this turns out to be a worthwhile investment is up in the air. Apparently, the current price difference for PPGA vs. SEPP 366s is less than $15.00, certainly less than the cost of what an adapter will sell for.