Hmm.. kinda like the annoying paperclip in MS Office 98/2000?

Eh.. its not a bad idea. I kinda like it. It could have random comments based on the current action.
But what say you generalize the whole idea slightly. Here's what I'd do:
Somewhere in the code, or even in a yyfunction (i.e. <yabbtips> on the template equals yytips I think) -- implement a DIV tag that has a mouseover action which pops up the appropriate text after checking the current action. A list of actions and the possible text strings for that action could be stored within a variable and within one of the board's MySQL tables, and made accessible through the Installed Mods & Settings panel:
Action=Random Text
BoardIndex="Isn't this a cool board?", "To go to a board, just click on the Board Title in the Index Page here.", "YaBB SE Rules!"
Post="To add smilies, just click on one of the smiley buttons above the message box.", "If you want to see what your message will look like, click on the Preview button."
InstantMessage="Don't forget to type the subject AND the recipient.","If you want to check the name, go to the memberlist screen."
Then, display a message box where they can type HTML that should allow the mouseover option, including image, text, link, whatever. This is the HTML which would go inside the DIV tag.
I dunno, something like that.