I don't know, some might find this useful. I always wanted to define a smart collection for all the images that do not have the capture date set. See the attachment for a definition that does that (it uses nested rules, which is a cool feature).
Smart Collections - True and False scenarios & Nesting
Mod Note: Mark, Thanks for the info, I merged your post into the existing closed thread to make things easier to follow.
With reference to the tip by Mixx on smart collections for photos with no dates I could not replicate the settings on the smart collection dialogue box for which a screen shot was attached. In particular I could not get the "True" and "False" scenario and the nesting of fields. I had to delve into the Lightroom help pages to find out what was going on and I suspect that not many LR users know about this feature.
To quote the help page :
Alt-click (Windows) or Option-click (Mac OS) the Plus icon (+) to open nested options that let you refine criteria by whether Any/All/None Are True.
When you open the Smart Collections dialigue box if you press the Ctrl key (PC) before clicking on the "+" to add another field the "+" turns into a "#" sign and when you click it it creates the "True" & "False" scenarios and starts the nesting process.
I have not yet had chance for a serious play with it but it looks to be an exceedingly useful tool.