Isn't this part of what the concept of "liquidity" is about?
Just because I cannot sell a hundred million dollar estate at midnight in the middle of a bank holiday weekend doesn't mean the estate is worthless, it just means such estates have somewhat less liquidity than, say, munchies or hits of "acid"...
-MarkM-
Well, in a narrow sense, for example, if you need medicine right now or you're going to die, and no one who has medicine is accepting Bitcoins/dollars/whatever, the Bitcoins/dollars/whatever are completely worthless. In a broader sense they are worth what they would sell for when more bidders are in communication with you (if there are indeed more bidders somewhere else)
This example also details how value is different for each person - there is no "real value" as the thread title suggests. If the person needing the medicine right now can't get anything for his Bitcoins, they are worthless to him, but if someone who doesn't need medicine can wait to sell them for other valued things, the Bitcoins are of great value to him.