I've observed the scientific community to be generally curmudgeonly about software issues overall, so I'd predict that they will be the last group to move to Python 3. So once every other community has moved over (in this order: hobbyists, bleeding-edge startups, tech companies/startups in general, corporate environments), then it will be their turn, and they certainly will because the ecosystem will have completed moving over.

As far as 2to3/3to2, that entire process in my opinion will be going away; once you've pegged Python 2.6 as your bottom version, it's straightforward (though not without effort) to produce a codebase that runs in Python 2 and 3 without any changes. I've already done this for my template language Mako, where version 0.7.4 has been enhanced to support Python 2.4 all the way through the latest 3.3 without any 2to3/3to2 step.