My daughter has a stock Motorola Atrix on Bell in Canada. When she receives a text from one particular friend (on a Telus phone), rather than displaying the text, the phone just displays "null". All other texts work fine. She can receive from other people on the Telus network as well.
When the problem contact sends an MMS message with a picture, any text in the picture message comes through fine.
I have tried multiple SMS apps to see if they made a difference but they all behave the same way as the default text app.