The only way I know of is via Connection Switcher, which is installed as part of Maemo-PC-Connectivity (believe it's in extras-devel). It's meant primarily for managing network connections via BlueTooth, USB, etc., and gets installed to the status menu.**

You can use it to create what it calls an "Environment" (called 'work', for example) that will allow you to specify the default state for USB when you activate it. Then it's a simple matter of switching to your 'work' setup before you connect the USB cable and it should only connect in PC Suite mode for you.

There are instructions for using it HERE.

..it's odd that you're not being prompted though, since that's how it works for me.



**Note that Mameo-PC-Connectivity is a suite of apps, so you'll also get a few other things along with it like an SSH server and some kind of VNC functionality I know nothing about. Probably a couple of other things as well.