The full release notes are on the Arcen forums. This release is actually a pair of releases, one released this morning and the other released tonight.
This release pair has a lot of important stuff in it, but it's nothing game-changingly exciting. The 3.181 release had an issue that would cause some long-running savegames to become absolutely unwinnable within a few minutes of being loaded (thanks to over-aggressive border aggression), and consequently these releases include updates to really revamp both the border aggression and the AI barracks for the better. There are numerous other balance adjustments here for the base game and CoN -- important stuff, but individually minorish tweaks.
The Intel Summary in the galaxy map has seen some improvements, as has the lobby for launching games, based on making things easier, fixing a few bugs, and preparing for the Unity 3D changeover. The ship category buttons along the bottom of the screen have also been revamped to have sub-categories now, which fulfills a few long-running requests. There are also some new warnings that now pop up when planets are under attack by the new CoN minor factions: hybrids, preservation wardens, and so on.
Lots of important progress and polish, in other words, but it's not as new-toy exciting as the last couple of releases. Still, for those players who were finding their savegames unplayable in 3.181 because of the AI incursions that was causing, this was exciting in a different way!