OK, so who knows the answer to Desmond's riddle to Locke: What do two silhouettes say to each other?" I know he was fishing for a password to confirm that Locke is indeed "Him", whoever "Him" is, but I feel the answer might be a clue.

I think most people believe it was:

What do the 2 snowmen say to each other?

There are several threads on this. The answer is - Do you smell carrots? or words to that effect

It was "snowman" and most likely the reply would be an unrelated phrase or question. Something like "Windy days are best for flying kites." That's typical spy agency "challenge" exchanges for identifying fellow agents or in this case, relief at the security post (at least in TV and movie land.) Such an exchange eliminates the remote but possible chance of guessing the correct answer to a challenge question/phrase.

I've seen another answer to the riddle being "Freeze".

Thank you, I learned later it was Snowman, not Silhouettes (I do have a bit of a hearing problem); also, I inadvertantly hit enter twice when I first established this post and cant figure out how to make one of them disappear. Moderator?
Freeze? Good one, Chelly. Nice to see ya again, by the way. Still love your avi.

I've seen another answer to the riddle being "Freeze".

That's what I thought the answer was. Makes sense.