If smscon is active (i.e starting on boot etc.), when someone inserts new SIM card, You'll get SMS send *to* Your master number specified in SMSCON (in case it was Your own number, You'll get it into N9, but You need to recall what You've put there), containing number of *new* SIM card inserted. So, you'll be able to control device.

It would be funny/awesome to catch burglars due to N900 and SMSCON. don't get high hopes, though.

/Estel

// Edit

Of course all of this will happen, *if* and only *if* You've set SMSCON to disable asking for lock code (*not* PIN code) when unregistered SIM is inserted (or, if You haven't used lock code at all). Otherwise, new "owner" will need to reflash FIASCO+COMBINED before using device, wiping out SMSCON.