I promise, I searched, but couldn't figure it out.
I downloaded nUI this morning. I reviewed the steps in the 'intro' email and followed all of them (renamed WTF and AddOn folders to backup, etc).
I have only nUI installed at this point - no other addons.
I have used the 'reset to default' option from within the chat windows (including the Combat window), I have used the /resetchat (or was it /chatreset ... whichever) to try and clear all the settings, then deleted and reinstalled nUI.
I ran the Repair tool, just in case! I've deleted WTF, Interface, and Cache and started from scratch. Logged in to verify things looked virgin, then installed nUI from there. No luck.
Repeatedly, when I log in for the first time after installing nUI, I get this:
Message: Interface\FrameXML\ChatFrame.lua:3946: attempt to perform arithmetic on a nil value
Time: 05/06/12 15:37:20
Stack: Interface\FrameXML\ChatFrame.lua:3946: in function `ChatEdit_UpdateHeader'
Interface\FrameXML\ChatFrame.lua:3671: in function `ChatEdit_ResetChatType'
Interface\FrameXML\ChatFrame.lua:3743: in function <Interface\FrameXML\ChatFrame.lua:3727>
Interface\FrameXML\ChatFrame.lua:3753: in function `ChatEdit_DeactivateChat'
Interface\FrameXML\InterfaceOptionsPanels.lua:978: in function `InterfaceOptionsSocialPanelChatStyle_SetChatStyle'
Interface\FrameXML\InterfaceOptionsPanels.lua:907: in function <Interface\FrameXML\InterfaceOptionsPanels.lua:894>