Pretty much what Sonic Tron said... that is assuming your talking about the standard 1 ghz Celeron with 128k cache. Probably around a PIII 800 or 850 mhz equivelent.
However, if you are talking about the 1 ghz Celeron with 256k cache, that IS a Pentium III in just about every way possible except for two things. There is no 133 mhz FSB version, which PIII would have, and SMP has been disabled so you can't make a dual processor system out of it. Otherwise, it should benchmark identical to a 100 mhz FSB PIII. The 256k cache variety is refered to as a Tualatin cause it's made on the .13 micron die process and uses less power and has good OC potential.