switching from unlimited data plan to pay-per-data. what to watch out? I have been using unlimited data plan, but it is about to finish and after that I'll pay my data charge per data usage. Any particular settings should I change or anything I should take note in order to avoid suddenly paying a lot of money?

My phone is Galaxy S with Froyo.
DroidWall is an application (for rooted phones) that will let you select which applications are allowed to use which data networks (WiFi vs. Cellular/2G/3G/4G).  You can use it to make sure that certain applications always have data access when needed and restrict other applications to WiFi (or even no acess).