Since Apple’s announce that Ipad and also the next Iphone 4 will use the micro SIM format for 3G connectivity, this micro sim is become more popular now and it seems the telco operator responded very quickly about it, they race to be the first to provide this new type of SIM card to become available for customer to use.
So what is micro SIM? Micro SIM also known as 3FF (Third Form Factor) SIM is a diminutive about 12 x 15mm, it is smaller than usual GSM SIM Card (15 x 25mm). Image on the right shows the evolution of the card based on ISO standard. So this new type of form factor will not compatible with your current phone, unless somehow you do the modification yourself by cutting it.
Below is the comparison between normal GSM SIM card (2FF SIM) and 3FF SIM, there is also a combi card (as you see in the below right picture) which has the 2FF SIM and 3FF SIM in a single card. Whenever you want to use the micro sim you just plug-out the card for the micro sim but after you plug out the card you can not move it back to original 2FF type.