I think the truth may be somewhere in the middle.
Take Scott Clemmensen as an example.
http://panthers.nhl.com/club/player.htm?id=8466339
In Jersey when Brodeur went down - he came in and posted a .917 sv% on a team that was very defensively sound. Then he went to the Panthers - a team that is nowhere near as good defensively - and posted sv%s of .912, .911, and .913. A minor drop off, but not huge. Obviously he's doing much worse this year, but whether that's due to the players in front of him or whether he's just playing lousy, or a combination of the two - can't really be determined in black and white.
I'm sure you guys can find other examples of goalies going from a team with strong defense to a team with lousy defense (or vice versa). Vesa Toskala maybe?