Ok so I got it working and did some basic testing and all seems fine. I'll update this later just to reconfirm this after some solid play.
Both Titan Quest and Immortal Throne seem very playable.
There is a caveat but its easily overcome. Strangely the main menu does not accept keyboard input for typing in the names of characters when you create one. The options screen does however so you can still bind your keys and what not. So dont worry about all that, what I observe so far is that it stops you typing letters in the name character menu.
But heres the trick. Before you launch the game type the name of the character you want to create (it can be anywhere) just copy it with CMD+C. Now then launch the game & instead of typing the name (As it wont let you) just paste it instead. And there you have it, you now have your character and are ready to play.
Alternatively if you've played this on bootcamp before like myself you can just copy your save games over to the ussual save game location
username/my documents/my games/Titan Quest - Immortal Throne
By the way in crossover your windows user name is "crossover"
There was an issue with textures not showing at first but all i did was disable V-Sync and a few other basic graphics option tweaks and all was perfect after that.
And thats that... Hope it helps someone, really getting into this game at the moment in anticipation of Diablo 3... of course.
The game is working just fine on both my machines
8 Core Xeon Mac Pro 2.8ghz
Geforce 8800GT
OS X 10.6.8
Crossover 10.3
Mac Book Pro i7 2.66ghz
Geforce GT 330M
OS X 10.6.8
Crossover 10.3
Like I say I'll chime in again sometime soon to let you know if I spot any bugs but so far so good