Did you add a copy of dgVoodoo in the Defiance directory? As hort_wort mentioned, Defiance won't see the version in the original game's directory. I should probably point this out in the original post.

The links to dgVoodoo seem to be down. Is there a mirror or backup version of the page where the dlls could be retrieved from?

The dgVoodoo homepage seems to be back up again. In case it goes down again, I'll put the file on a mirror for you. Just shout.
Perhaps GoG wants to consider to put the file on a mirror of their own, it's a very fine resource for one or more of their games after all.

There seems to be two versions of v1.50: Beta and Beta2. Which one is considered more stable?

I tried both versions. For me it made no difference. I run a Core i7 and a HD5870.
Perhaps the nVidia users have experienced differences.

OH SNAP! This is precisely what it needed to run in CrossOver Games!
I installed the dgvoodoo patch, set the resolution to 800x600 and windowed mode, then ran the game with flags of -w -800x600, and it works! Perfectly!
I had an issue with the display being off the bottom-right corner of the 800x600 window, but I just dragged the resizing handle in the corner a bit to make it big enough, and I can see everything. I need to disable OS X's grab of the F-keys, though, since it makes it difficult to use autopilots.
Now it's time for some hardcore tweaking to make this thing run at optimal performance...
I think I ought to start a thread?

He did. I-War is now playable with nGlide 0.94 too. :)

Sweet! Thanks for the heads-up.

I've not tried tweaking any settings, but by default it seems to run awfully slowly for me by comparison with the alternatives, and the movies seem to be a little funky (I think the gamma value is too high). It's certainly a huge improvement over the 0.92 version that I tried last time, though. I'm glad to see that it's still actively being developed... maybe more improvements will come over time.

I've tried using this for I-War, but as soon as i'm on a game screen the games becomes really slow. I have huge mouse lag and it disappears when i'm in a non-game menu.

Actually nevermind, it was my fault, i had turned star blur on.