Sorry, couldn't resist.

For the keyboard to open and unlock the screen it has to be fully opened, which never happens for me. Also if the phone is locked, then opened and then shut it will re-lock.
What normaly happens to me is I forget to lock it and that is where the problems happen. Having had the phone for 18 months, I am now good at locking it.

I don't have CSSU as Modest (email) has a lot of problems and they have re-introduced old, once fixed bugs back in which I fine annoying.

You could write a little script using a dbus-monitor command to catch the keyboard opening and then lock the device again, although I think I would find that far more annoying.