If you were using a Nexus 1...it would be solved like this:
Here's how I've gotten the default to change:
get a task manager (I have Taskiller)
start up the calendar and change to the view that you want to be the default (Week, for you)
hit Home and start up the Taskiller, kill Calendar
next time you start up Calendar, and any subsequent time, it will start as the "new" default
Since we have droids...we solve it like this:
(1) Open the calendar app and set it (using the menu) to the view you prefer.
(2) Use the home button (at the bottom of the Droid) to exit the calendar app, NOT the back button.
(3) Reboot the Droid. (Long press the power button, select shut down, then restart the Droid).
This will leave your calendar in the startup view you want, and the back button will take you back to the home screen, not another one of the calendar screens. You can change the view mode all you like, and hitting the back button will step you back until you get to the startup mode view you are using, with another back press taking you out of the calendar app. It will remain this way until you (deliberately or accidentally) use the home button to exit the calendar app, and subsequently reboot your Droid.
Not as good as a preference, but works fine for me.