Intel isn't trying to reserve the Slot X market for PII's, it is just that that dinky little card that the celeron is currently built on raises the price, both for Intel to manufacture and for the big OEMs to integrate (it's harder to build a robot to plug the card in, make it firm, etc. than to just plug a PGA chip in to a socket). The socket makes it easeier for OEMs to make computers with 1 giant board that has all the components integrated on it (ick) -- somethinng that is very popular with P5-class machines.

BTW, the LX and BX chipsets were always designed to run Celerons as well as PIIs. The EX chipset (the one the poster was trying to think of) was designed to be a very low-cost, Celeron-only platform (it leaves out SMP, half the DIMM sockets, and some of the PCI and ISA slots), but I think Intel was the only company to make a motherboard with it (a crappy little thing, but you could buy them in bulk for about $40 each).