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.
Related to : SMS messages just have "null"
delete their contact
Settings > Applications > Manage Applications > Select the Messaging App
Choose 'Clear Data' and also 'Clear Cache'.
N.B. you will lose all data / txt msg's in your phone
(don't forget to write their number down to save as new afterwards!)
You could also check on http://www.google.com/contacts and check if the address book entry for that person is missing some information.