Finally! Someone else has the same problem.
I confirm, Patchou! I have the same problem (that's why I don't use CN).
It does not show neither the original or the custom name while chatting, and unchecking/checking the "Show Custom Name while chatting" option does not change this (yes, I close the chat window).
About not showing the original nick, I guess it's because the option must be "show contacts by email" for the Custom Names.
I have:
MPlus!: 3.25.106
MSNM: 6.2.0137
WinXP HE
Edit: SP1
Edit 2: Solution, at least for me, found
here.
If you're using the timestamp feature in StuffPlug-NG, this is the cause.
/ 
