From the 'speedy release' files:

Mozilla developers have pushed out Ubiquity version 0.1.5, the second update for Ubiquity in 2009 (I know!!). The 0.1.4 release was out on Jan 6th while 0.1.3 was out on December 30th.
 
The big deal with 0.1.4/0.1.3 was new skinning capabilities for the semantic web tool.

The 0.1.5 release continues that work. According to the release notes:

 "Ubiquity 0.1.5 is about stability and reliability release. It's much more stable than 0.1.4 was, and has a wonderful new look that works across all platforms."

So basically - a bug and stability fix which is what you'd expect from alpha software.