From a reader:

finally, a topic on the corner that i know something about…
the multiple universe (multiverse for short) scenerio can arise in many different ways, some more esoteric than others. there is a neat hierarchy of multiverses due to max tegmark (a cosmologist at MIT):
level 1- multiple hubble volumes. what physicists generally mean by the universe, is the observable universe. this is about 15 billion light years in size (that’s 90,000 billion, billion miles) and contains everything that we can “see” (observe would proably be a better word here). however, we have reason to believe that our hubble volume (hubble was the chap who first observed the expansion of the universe and “hubble volume” is just another way of saying observable universe) is only one of many (many could be 1 with a hundred zeros after it, or even more). in these different parts of the universe the laws of physics etc. are al the same.
level 2 – string theory, as your earlier correspondent pointed out, gives another possible source of multiple universes. the reason this is different from level 1, is that in such universes the fundamental constants of nature (for example the mass of a electron) are different, though the underlying laws of physics are the same.
level 3 – contained here are the universes resulting from the many-world interpretation of quantum mechanics. i don’t want to launch into debate on the merits, but i would point out that it is not an unreasonable way to reconcile some logical inconsistencies in out understanding of the quantum world. this, i think, is what you had in mind with your original post.
level 4 – as well as universes where the constants of nature change, one can also imagine those where laws of physics are different. more precisely one could believe that every possible consistent mathematical structure leads to some kind of universe.
it’s worth remembering that while levels 2 to 4 are speculative at best, level 1 isn’t. the enormity of the universe, and the existence, therefore, of many observable universes, is a consequence of the theory of inflation. this is description of how the universe behaves in its earliest stages (as the name suggests it undergoes a dramatic, and very rapid, expansion), and we are fairly confident it is correct – it is certainly consistent with all known experimental data. hence, as uncomfortable as it may make you feel, i’m afraid at least part of the multiverse is probably here to stay. furthermore, i’ve heard it argued (though i’m not sure i believe it myself) that it could well the case that there are sufficient numbers of observable universes that an appreciable fraction of all possible configurations of stuff take place. so that, in fact, there is a hubble volume where you chose the bud light.
[Name withheld]
ps i suppose i should list some credentials – i’m a phd student in physics at stanford and among other things i study the string theory landscape (level 2).
pps if you really want to get into this stuff there’s a marvellous new popular book on the subject of the multiverse by leonard susskind (one of the inventors of string theory) called “the cosmic landscape”. as one of the architects of the current resurgence in multiverse ideas he’s definitely worth reading on the subject. also he’s one of my profs.