Ok dumb question: I know you can create multiple addresses for receiving coins, but when you send other people coins, what address does Multibit use as the "sending address"?
I am not familiar with the internals of MB but as coin age plays an important role in the transaction priority, the satoshi client will pick an old address. Also the client most likely tries to optimize the transaction size by taking fewer inputs, so when you have addresses with 0.1, 0.2 and 0.3Ƀ respecitvely, it would take the 0.3Ƀ address when you try to send 0.27Ƀ rather than the 0.1+0.2.
If the 0.3Ƀ is less than 3 days old though, it would have less than 1BTC-day of age and would need a transaction fee to get processed, so the client might pick the older coins.
If you want to control that, you need a feature called "coin control". Again I'm not sure if MB has that.