I think the best solution would be an app to disable data connections when in X mode.
I used to have this problem on 2G where if I was in data mode I would never get incoming calls. This seems to have stopped now - I think this was a carrier issue not the n900.

When you get this problem have you tried disconnecting your "internet connection", as this should then go back to 2G (as 3.5G and 2.5G are data connections)

I'm glad I'm largely 3.5G now.