Ok, I'm evil, I admit it. The joke is complete gibberish to anyone who isn't a scientologist (kudos to debra) because it uses specific scientology terms.
Now of course, using scientology terms in front of wogs (non-scientologists) is a bad no-no. Good thing I'm no long a scientologist (aka I'd probably be classified a SP, which ranks alot lower than a wog). A Thetan is what they call the spiritual being which is you (off of the greek theta which means spirit). The crux of the joke lies in two parts. The definition of screw (aka - have sex), and the definition of 2-d, or more precisely a term they've coins called 'out 2-d'. In scientology there are 8 dynamics (or ways of classifing groups in relation to you). The 1st being the individual, and the 8th being well .. everything (which has also been perceived as god, since god is everything). The 2nd dynamic (called 2-d for short), deals with the individual and his family or relationship/spouse. If someone is 'out 2-d' then they are having a problem in the second dynamic (exampled by less or more than 2 people involved in an intimate relationship, since you are only supposed to be with one other person).
To rewrite for a more xian take on the joke:
How many Christians can screw in a lightbulb? 2, otherwise they are sinners going to hell.
It's a lot funnier the scientologist way, but only to a select group. Now you have a defense, if you see them, tell them the joke, and when they bust a gut laughing, run away really fast.
-Munch "I have to say, lickerish is the best response I've seen so far ... although strawberries and cream just causes my brain to frittzzzzzzzzz..." Wolf