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I think this is a great idea as long as the client is satisfied with your design. The only drawback is if the client wants to move several design elements around or other major changes.
In Photoshop or Fireworks this could be accomplished much quicker than it would re-styling/coding the mockup again.
I’ve often done something similar with IE such as giving rounded corners/drop shadows to non-IE browsers while giving IE users a less glitzy website.