Well remember, there are many CPU architectures, so clock speed becomes irrelevant to certain respects.
Desktop Processor Family/ Number Sequence
Intel® Pentium® 4 processor (including the Intel® Pentium® 4 processor supporting Hyper-Threading Technology and Intel® Pentium® 4 processor with HT Technology) 5xx
Intel® Celeron® D processor 3xx
Mobile Processor Family/ Number Sequence
Intel® Pentium® M processor 7xx
Intel® Pentium® M processor Low Voltage (or LV) 7xx
Intel® Pentium® M processor Ultra Low Voltage (or ULV) 7xx
Mobile Intel® Pentium® 4 processor (including the Mobile Intel® Pentium® 4 processor supporting Hyper-Threading Technology and Mobile Intel® Pentium® 4 processor with HT Technology) 5xx
Intel® Celeron® M processor 3xx
Intel® Celeron® M processor Ultra Low Voltage (or ULV) 3xx
Same thing with AMD chips, where now they're slowly phasing out the 754 socket CPUs as a lower-end user range while the higher enders go for the 939s.. so you hear people saying 'you can actually choose different types of e.g Athlon 64 3000+??'
If you think the CPU designations are confusing, try learning ATi and Nvidia's notation for their video cards. ATi recently ran out of room for their Radeon cards (upto 9800XT), so they started the X300 up to X800XT range.. and some people got pretty confused.
Same deal with the Nvidia FX series (after the Geforce 4800 Tit), they suddenly introduced the 6800.
It gets pretty annoying eh?