Some mobile operators offer a multi-SIM service that effectively "clones" your SIM card, allowing you to use multiple phones and PDAs with multiple SIM cards that are linked to a single mobile subscription.
You may make multiple phonecalls and mobile data connections simultaneously. However, only one of the devices can be designated as the master device to receive phonecalls and SMS messages.
Why is multi-SIM useful? Every day, I carry with me five mobile devices that require SIM cards:
(2) Nokia N82, my primary work phone
(3) iPhone 3G, for quick web surfing and email access
(4) HTC Touch Pro, for keyboard-intensive tasks like photo blogging
(5) Nokia 2650, a phone without camera that I bring to workplaces where cameras are prohibited
I used to swap SIM cards on these devices, which is quite tedious to do while on the move. Now I'm using a multi-SIM on my laptop and iPhone. I don't need to swap SIM cards any more, and both devices work without problems. Overseas data roaming worked fine too, when I tested my iPhone in Japan and Canada.
So if you need a new SIM card for your mobile device, talk to your mobile operator about getting a multi-SIM.