It's rare that they'll interfere, even in close proximity. I try to keep them out of the same pocket, but often have my cell on my belt on my right hip and the FOB in the right front lower pocket of my coat and they don't interfere. I think I experienced some bluetooth interference once when the fob and phone were nearly touching, but that was it.
my husband has had a problem with
the 2004 not finding the smartkey
when the key's signal has been
blocked by his iPod in the same
pocket... I think that the phone
has done the same too. move the
object out of the way of the key,
and it works fine.