If you have bought yourself and iPhone 4 and have been trying to use the SIM card on another phone, you might not be able to do so because of the new MicroSIM being used in the newer iPhone devices.

However, with technology there is always a workaround to a problem. In case of the MicroSIM in iPhone 4, you will be able to use it on another device or an older iPhone using a new MicroSIM adaptor accessory which has a plastic form factor into which you can put in the MicroSIM and use on an older iPhone or any other phone which supports the older SIM card.

The device which is sold at the MicroSIM shop retails for €5.99 (~$7.5) and is sold through Austria. The adaptor was put through tests by the Unofficial Apple Weblog and it did work pretty well. So if you have been itching to get back to your older phone, but cannot do so because of the MicroSIM, go ahead and order the adaptor to put an end to the antenna reception issues, till it is fixed by Apple.

(Source)