I found this through my blog traffic stats--thanks to whoever linked me :)
@Joze: A trilogy has a very specific structure and to be honest, it's blatantly obvious when a narrative is over-stretched to reach that three-book sweet spot (this is happening more and more as the popularity of the trilogy rises). A story needs three "cycles," each with its own climax, to function as a trilogy.
You think your story will be longer. It's very common in fantasy to write ongoing series. Here are some of your options:
1) Trilogies typically follow one character and their issues. Canavan did this, as you menton; Jacqueline Carey also does this successfully. Each book in this trilogy focuses on a different part of the journey, each book resolves that certain part of a journey (while posing new questions), and each book offers something new. When the trilogy is over, our character has closure. JK Rowling got away with stretching this into seven books because each had a strong self-contained storyline, and her ensemble cast was utterly engrossing/deserving of attention.
2) Ongoing series, each dealing with a different character in the same world. Anne McCaffrey's Pern series is a great example of this. These stories tend to be self-contained and there is no ongoing storyline which carries the entire series. You could pick up any book and dive straight into the world; it's accessible, and accessible sells.
The Wheel of Time-style dragging sagas...not so much :P due to storylines that frankly take too long to meet their climax. Remember, nobody wants to wait an indeterminate amount of time for that all-important resolution (hence trilogy popularity).
3) Ongoing series fronted by the same character a la Bernard Cornwell's Sharpe series. Again, the storylines are self-contained, but the characters do tend to grow stale after a couple of books.
Conclusion: if you have one loooong story to tell, structure it into a trilogy (and remember that in fantasy, your word limit is a bit more elastic, so don't worry about feeling restricted. 120k would pass for a very strong book). It'll be a lot easier to handle than a long series with an ongoing story and an easier sell.
If you have lots of stories to tell about various characters, structure as a series that will offer each their own book.