You will certainly have noticed that since the passage in firmware 2.0 [iPhone OS 2.0], iTunes takes care of launching systematically a procedure of backup full of your iPhone right after the synchronization of the last.
This procedure is relatively long especially if your iPhone is full. It is a pity that we cannot currently establish a backup schedule customizable.
In the meantime, if you don't need to perform systematic backups, there are a tip on Mac to disable this feature.
We discuss it on the forum.