Because its carrier locked to AT&T and no the smart sim card is not complicated its actually a rather easy method(well at least for prepaid sim cards)
1*You take out your 3G sim card out of your iPhone.
2*Cut a portion of the prepaid sim card with a sim cutter(it makes a hole in a part of the sim card that wont damage the prepaid sim card)
3*put your smart sim card and your prepaid sim card together in the slot and thats it.
This i'll basically trick the iPhone into thinking its still with the same provider and let you use the prepaid sim card thats in the iphone.
If you wanted to sign up with another network on a plan i think it would be just like any other phones, just place the smart sim card instead of the 3g one.