The first time I saw Louise Lombard walking in an episode of CSI, I thought she was wearing her gun shoulder strap too tight and it was affecting the way she walked. But after watching the video clip that szabowins just posted, it's clear that she walks that way regardless of whether she's wearing a gun.
She walks like a bodybuilder with her upper body very stiff. However, it does exude an air of confidence and intimidation that suits the character she's playing.
Does she have any background in gymnastics? Sometimes female gymnasts walk with a stiff upper body. Now that I think about it, her walk resembles that of Olympic gold medalist Mary Lou Retton.
The OP compared her walk to John Wayne. I thought his walk was much more relaxed in the upper body. But I do think that her walk does have some generally manly characteristics (upper body wise).

Here is a video where you can see her walk "that" way.
http://ca.youtube.com/watch?v=EGXyxPE7ZHc
I hope everyone likes it.

Eh? Do you mean she has a penny clenched between the cheeks of her arse?
When I die, I want to come back as that penny!

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i must agree. her walk is HOT!
i love a woman with a badass swagger.

I LOVE THAT WALK!!! its hilarious! (not in a 'haha you're deformed - you have bent arms' sort of way, more of a 'haha your character has a silly walk' kind of way)
i've noticed that Sara's started doing it slightly, but i watch the episodes completely out of order, so im probably wrong
(im in the UK and i see random episodes on Living TV, then watch the odd [more recent] one on Five)
but that's just me.

I've always held that the belief that the typical midwestern/californian american accent has a lot of irish in it, nice to see 238I'm not the only one. I think that Louise puts a little bit of east coast/mid-atlantic states into her american accent. It adds a little authenticity to her, as I think that's supposed to be her character's background. I think it's good, most American's would never guess that she's a Brit.

She's British, born in England
Sara: You weren't in your office.
Grissom: And good morning to you too, Miss Sidle.