Subscriber Identity ModuleOne of the key features of GSM is the Subscriber Identity Module (SIM), commonly known as a SIM card. The SIM is a detachable smartcard containing the user's subscription information and phonebook. This allows the user to retain his information after switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM. Some operators will block this by allowing the phone to use only a single SIM, or only a SIM issued by them; this practice is known as SIM locking, and is illegal in some countries.
there are smart fones/pda phones out there have the dual sim feature and both sims are on simultaneously, so yu can receive a call from either sim at the same time, its pretty uber ill find some links.
the manufacturer's didn't forget..( evidenced by the fact that when the phone senses the dual sim adapter, it gives you a menu option)is just that now, the SIM is what holds the data now, not the phone..... also, seeing that the phone company usually subsidises the cost of the phone substantially, they wouldnt really want to allow you to use the equipment on another network , jus so .. ( which is why they 'sim lock'). the manufacturers probably sided with the phone companies