I'd normally be in favor of a large warning, but we are talking about preparing for an onslaught of average clueless users here.  The average user is not going to read a warning of the length that would be required to sufficiently describe why it's a bad idea, so they'll click OK and still get angry when they don't generate anything.  Thus, keep the text of the menu option but make it pop up a very short message.  Something like "You are unlikely to generate a block in any realistic amount of time on a standard CPU.  If you do still wish to enable generation, download the separate mining client at www.example.com"

Personally I've mostly stopped mining entirely.  The difficulty is so high that even with a pool, using my GPU and both CPU cores, it would take multiple days to earn 0.20 BTC.  Even when my electricity is free, that's not worth the additional strain on my computer's components.