I have only seen this occur with one of my contacts and I have noticed it two times in conversations with the person.
Basically, everything is fine for a while as I talk to the contact until out of the blue (I dont actually see the change, another window open) the nickname that was in the tab changes to the email address of the person. This person did not change their name.
The nick stays as it should in the rest of the conversation window, it is only the tab that this occurs.
Windows Version: Windows XP, 5.01.2600
Messenger Client Version: 7.5.0311
Messenger Plus! Version: 3.61.0145
Feel blue skin also installed