Ive found that not to be the case...
ive got a ata100 ibm 60GXP and a ata33 ibm 16.8GP as primary master and slave.
both go at their designated speeds (done the benchmarks to prove it).
the only thing to look out for is to make sure the faster drive is the master and to use an ata66/100 80 pin cable as opposed to the older ata33 40 pin cable.
oh yes... also helps if your motherboard HAS an ata66 or ata100 controller too!
My Hamster has 512MB of SDRAM @ 150Mhz CAS 2!