After upgrading to iOS 4.1 you may experience this simptoms:

1. Opening emai message you will see the message body empty.

2. Composing new email you will see no message body field.

Solving this situation is easy - just couple of touches of your iPhone/iPod screen:

1. Go to Settings->General->International->Language

2. Pick any other language that is different from the current one and press "done".

3. Go back and select original language once agan.

After that mail messages will be displayed as expected and you will be able to compose emails normally.