I welcome discussion on this point.
I don't think it is as sensitive or as important a topic as the author order on papers.
With regard to paper authorship my general rule is that the person who does the bulk of the work, including actually writing the paper should be the first author.
Doug Natelson has a good post on co-authorship, that I largely agree with. My only difference is that I am not really convinced that good practice prevails in the majority of circumstances. I fear there are increasing numbers of co-authors, particularly senior ones, with marginal contributions.
But, what about conference talks and posters? Many of these are based on work that is already or about to be published. Should the author order be identical as the associated papers? I am not sure it should necessarily be. My tentative view is that the person who writes and submits the abstract and actually prepares and presents the post/talk should be the first author. Perhaps they should also highlight their own contributions in their presentation.
I am also not sure you should have co-authors of a conference presentation who have not looked over the abstract and the presentation beforehand.
What about conference proceedings? I think they are a waste of time and should be abolished.
I am not sure this is a particularly important issue but it may be worth discussing.