http://cvs.gis.umn.edu/dist/mapserver-4.8.0-beta1.tar.gz snuck out the other night.
Lots of bug fixes in the new version. The major new features are
Bilinear and averaging raster resampling options
Better transformation of vectors at the horizon for orthographic projections
New layer plugin architecture that allows dynamic loading of custom data drivers
For Python users, this last one implies the option of custom data sources implemented entirely in Python. I'll be working on this very soon.