Changing affiliations and conference registration Say the deadline to register for a conference is now, but the conference doesn't actually run until 3 months from now.  During that time, I will have changed institutions.  When I register, should I use my current or future affiliation?

There have been several questions here about how to deal with changing affiliations on papers, but I think the nature of my question is different, since there isn't a paper or publication at stake, per se.
There are no hard and fast rules here, and the conference organizers almost certainly do not care.

For most circumstances, the only thing that matters is this: which one do you want people to think of you as being from at the conference?

Most of the time, this will be your new institution.  If you know you're going to be there when you register, why not just give that affiliation?
Otherwise, you may end up doing what I've often seen people with recent changes do, taking a pen and modifying their affiliation on the spot.