Subject: Does the Casimir Effect only work in a vacuum?

Every physics textbook that i've looked at seem to say that the area in 
which the two highly polished, very close together metal plates are in, 
has to be a vacuum for the Casimir Effect to work.

Why is that?  From my understanding, only particles of certain 
wavelengths "fit" inbetween the plates and push out, while the rest of the 
waves (obviously more) push in.  Surely if the air around the plates was 
still i.e. no wind or atmospheric complications existed, then the same 
would apply i.e. more waves outisde the plates pushing them together than 
pushing them apart from the inside?

Or, on the other hand, is the whole point of the Casimir Effect to prove 
the existence of virtual particles i.e. a vacuum is not really 'nothing'?

thanks,

p.s. i've seen the other queries about the casimir effect, but none seem 
to say that you definately need a vacuum... just to let you know that i 
did check these pages first before submitting this!

Re: Does the Casimir Effect only work in a vacuum?