How does the iPod prevent people from knowing about Macs? Owning an iPod makes them MORE likely to know about Macs--and one day consider owning one. Not less.
Not every iPod has to convert a user to Mac instantly--that's not how brand awareness works. If even 10% of Windows iPod owners decide, in the next 3 years, to try a Mac, that's pretty significant.
One could argue about how BIG or how FAST the "iPod Halo" effect is--I'd answer "medium" and "slow"--but you can't argue that it's bad for the Mac platform.
Besides, simply selling Macs directly isn't the only benefit the iPod gives the Mac. It puts Apple in journalists' spotlight to make them pay attentiojn to other products--like Apple cleverly arranged with the iMac G5-iPod comparison. It counters attempts by Microsoft to dominate another industry--which would be bad for all other platforms. And it brings Apple some cash
All good.