Hmm, I haven't encountered this problem, but this is what I would try in your shoes -
First, and I'm not sure I'd be real optimistic about it working, I'd try playing in windowed mode - everytime I start my client in windowed mode with a resolution smaller than my desktop the game window resets to the top left corner of my screen (I know this because it bothers me that I always have to drag it down and center it).
If that didn't work, I might try setting a custom resolution in in the ini with a really wide horizontal resolution, thinking I could maybe see enough of the window to grab it and pull it back into view.
Also, once the game is running in windowed mode, I might try opening the task manager, minimizing the game via right click, and then maximizing it - for me, that opens the game fullscreen with a black box filling in any empty pixels outside the game window. You could play that way, but my hope would be that after quitting it would reset its position and I could get it working right.
Worst case scenario, reinstalling video drivers might be an option, or screwing around with my desktop resolution and THEN starting the game might trick it into opening on-screen.
-HD