Ian Lyons has written a tutorial on changing Lightroom's camera defaults:

Whilst I briefly mentioned the new “Set Develop Settings” command in my review of Lightroom 1.1 I didn't think it was necessary to go into detail. Little did I realise that some two months after 1.1 was launched that many Lightroom users would still persist in creating and applying Preset develop settings customised for their particular camera etc. Don't get me wrong, presets still have their uses, but having to filter images according to Model, Serial Number or ISO before in order that the appropriate develop preset can be applied is both time consuming and in many instances unnecessary. My guess is that users either don't realise that Lightroom is already capable of automating much of this work or don't understand how/when to use this feature.

Right now, you can only change the default for a particular camera / serial number / ISO combination. That's useful enough, but just imagine when you can change the default for a particular lens. One of my three most frequently-used lenses is getting on a bit and has less contrast than the others, so I always seem to pump up the Blacks slider for shots taken with it. Trapping a camera / serial / lens combination is a bit more complicated, but it'll come. And probably sooner than I change that lens.