I don't think there would be any problem factoring in the 
consideration that some models just probabilistically imply the 
creation of life. That's a fairly simple technical move. What is 
philosophically more tricky, however, is to decide what to do about 
the fact that different models would have different numbers different 
types of living things. Would my existence be more probable given the 
universe contains many observers than given it contains few 
observers? The assumtion that it would be more probable if the 
universe contains many observers is the Self-indication axiom. In my 
paper on the Carter-Leslie Doomsday argument, I give some arguments 
in favour of the negation of the Self-indication axiom. The bad thing 
about holding the negation true, though, is that it makes it hard to 
see how to escape the conclusion of the Doomsday argument.
> Two possible theories, as I mentioned, are first that you create all
> possible universes with all different variations on the parameters
> we observe.  This is probably a pretty simple theory.
The problem with this theory is that our univers is not what we 
should expect it to be if this theory were true. If all locally 
possible worlds are instantiated eqully numerously, then we should 
not expect to find ourselves in one that is so highly ordered as 
ours. For every possible world like this one, there are countless 
ones where pink elephants sometimes materialize out of nowhere or 
random  blue blobs form over houses whose street addresses are prime 
numbers. If all these possible worlds were actual, then why are we 
not in one of these irregular ones?
_____________________________________________________
Nick Bostrom
London School of Economics
Department of Philosophy, Logic and Scientific Method
n.bostrom@lse.ac.uk
http://www.hedweb.com/nickb