Stop iTunes from creating a local library on start up
I've had this happen as well. 

The best solution is not to let iTunes open unless the drive is connected, but I see how that isn't always possible when on the road. 

I found that (in OS 10.6 and iTunes 9, at least), if you make the MUSIC Folder in your home directory (~/Music/) read-only, it will not be able to automatically create an iTunes directory in the default location, and thereby it will prompt you to find the iTunes library. 

As an alternative, you can hold down the Command key when opening iTunes, which will prompt you to connect to a Library. 

Alternative 2: If you make a Symbolic link to the iTunes Library in your music folder, it will attempt to mount the drive in order to find it.