Do not install aptitude 0.4.4-4b+1!
Apparently someone decided to resolve the dependency problems with aptitude in unstable by doing a blind recompile and NMU of the package, presumably because of my horrible laziness in not doing an upload to unstable yet. The problem, as I wrote in the bug reports regarding the need for a recompile, is that this produces a package that's totally unusable, which is why I didn't do it myself. If you are running unstable, you should either not upgrade the apt system, or install the experimental aptitude (either using "apt-get install aptitude/experimental" or by going here). These packages are not perfectly stable, but they also aren't totally unusable.
I apologize for the inconvenience -- if I can get some time this evening I'll rebuild the aptitude in experimental and throw it kicking and screaming into unstable.