MediationThe latest issue of Psychological Methods (Volume 12, issue 1) has two (count them! Two!) articles on mediation and moderation.
The first, by Jeffrey Edwards and Lisa Schurer, "Methods for Integrating Moderation and Mediation: A General Analytical Framework Using Moderated Path Analysis" (link is to the abstract only) is about how to most appropriately combine analysis on moderation and mediation. The article looks a bit tricky - there are lots of equations, but they are not nasty hard equations, and working through them gets what you want.
The second is by Scott Maxwell and David Cole, Bias in Cross-Sectional Analyses of Longitudinal Mediation. is possibly more important, and is about whether mediation analyses are biased, if everything is measured cross-sectionally (i.e. at the same time). The short answer is yes.
This is problematic for many researchers, who measure three things, and then carry out a mediation analysis to show that X causes M and M causes Y. Almost every effect that we are interested in turns out to be biased if you don't measure the effects longitudinally.