Swep, if a certain thing is 'in doubt' or 'doubtful' then the probability is that it may not occur. For example "It is doubtful that Painless will ever drink anything alcoholic other than Barcardi", or, "A Blue victory in the North Africa SEOW is in doubt".
Therefore, by using the title "A joke about 'doubt'...", I'm able to play with the concept of doubt by using the follow-up "...or is it?" thereby inferring that the joke about doubt may in fact not be a joke at all (i.e. it's in doubt)...which is the joke. Sort of.
I'm glad to have been able to clarify this for you...
Jabo