I've wondered this, I think they'd be OK, but I know that I probably wouldn't believe it whatever the result.
I read an ebay listing saying that the out of date clearblue digitals would still work, there's just a slight chance the battery could be dead.
I did a clearblue digital today, even though there's no way I could be pregnant, but I'm apparently mental. So I took it apart afterwards and put the battery aside in case I ever found a use for it. I dunno if it would be possible to replace the batter of an out of date CB digital before peeing on it? Hmmm