With regards to experiencing an absence of hostile mobs (as this post is originally about), I have a few things to say about the potential cause.
Some time ago, it was sung around the block that the code for mobs in SMP had a hard limit on the total number of naturally spawning mobs counted within anyone's render distance. The code distributes this number between the players in the game, but it may not do so evenly. As a result, some players may be surrounded by mobs looking for hugs, while other players may only see 1 or 2 forever alone mobs. Our town (and likely other active towns) in test survival is always surrounded by hostile mobs, and we have a sh*tload of pasteurized neutral mobs. If what I stated was true, then much of the hard limit is distributed between our towns, leaving only a small number for St Theo to deal with. The theory I presented, however, was introduced before Notch stated he had redone the mob coding to increase the spawning rate, so it could be obsolete.
I pose a question in reference to another theory. Were you continually moving in mostly straight path in the wilderness, St Theo? In my experience, I found that mobs don't usually spawn ahead of you when exploring uncharted land. If you were to retrace your steps back, however, you'd find yourself surrounded in no time.