When you are programming on the iPhone with, or without, MonoTouch, you need to pay attention to what you are doing with regards to accessing device resources.  I recently wrote a blog post about background processing with the iPhone and specifically with MonoTouch and location based services.  Having worked with this for a while, I wanted to mention that you will want to be careful doing anything like this.  The reason is that the more you bang on services on device, you will use up the battery of the device.  In my case, I was going off device on changes, which was killing the battery even worse.  Bottom line, be careful when you use services and how you use them.  If not, you can use the battery up fairly quickly.