How many times have you started a long iPhone 3G backup only to find that somebody has called you and canceled the entire process? You can prevent this from happening. Just take the SIM card out of the iPhone 3G. If you have another phone that uses SIM cards available, you can put the SIM card in that phone while performing the backup. Finish your backup, then when it’s done, pop it back in.

Source: iPhone Tricks You Might Not Have Known