This gets at something I never understood about the start-with-PSD school of web design: if it's not possible to get the effect you want in a browser, what is it doing in a mockup?
For the record, I think the ideal situation is to sketch out your design with pencil/paper and agree on a general layout before touching any code.
When dealing with clients though, unfortunately they may expect to see pixel-perfect photoshop mockups before agreeing that a design is what they want, which can complicate things.
The reason you mock up in psd (gross) or fireworks or a sketch book (yay!) or anywhere is because you can be purely expressive in an orthogonal capacity than you would be if you were implementing in code from the start.
You ignore issues like "oh, will this p be in a div or in a series of lis contained by... blërg" while those implementation details are valid to keep in mind, they're not entirely crucial to address when you're sketching which is really the key word here.