The main problems I see in overclocking a laptop (other than discovering the proprietary mechanisms for altering FSB and multiplier) are heat and battery life. If overclocking a laptop were to be tried, I would suggest ONLY upping the FSB and memory bus while leaving the CPU at approximately the same speed. (i.e. Mobile PIII 850's use a 100MHz FSB and memory bus on the i440BX chipset, change the bus to 133MHz while lowering the CPU multiplier from 8.5 to 6.5) This would give you the significant advantage of the increased memory bandwidth without too much increase in heat and decrease in battery life as the processor will then be running at 864.5MHz, not much faster than originally. At least that's how my theory goes, I'm not about to trash a $4500 ThinkPad trying to test it. There are many obstacles (are the PCI/AGP and FSB frequencies linked, will So-DIMM's work at 133MHz, can the CPU multiplier be altered, etc.) to overclocking a laptop and it will likely be more trouble than you want.
Aaron Meyer
PS. Some mobile Celerons use 66MHz FSB and some use the 100MHz FSB, the newer models from manufacturers such as Dell and IBM will almost certainly have the newer 100MHz FSB Celerons, while "value" laptops based on the Celeron and not the K6-II+, will likely have the older (and cheaper) 66MHz FSB Celerons.