There is no formal release plan yet, but MapServer users can depend on another release around the same time as this summer's OSG05 conference. In anticipation, I recently reviewed the MapServer bugs that I own to adjust priorities and set targets. There is not a lot that I intend to do for the 4.6 release: bug 1061 extends symbolObj::getImage() to vector and truetype font symbols as well as pixmap symbols, and bugs 1128, 1203, 1209, 1266 address makefile and threading issues for the Java mapscript module.
There's a roughly 50% chance that I will also tackle the issue of non-square pixels for mapscript in April or May. This is a feature that is currently only available through the mapserv program's WMS dispatcher, but needed by a few in-development scripts and modules such as the Python Cartographic Library.