The game informer has this....
Finally, pressing up in the compass menu turns your gaze up toward the heavens. In previous games, astrology played a large role in character creation. Though Skyrim abandons the class structure in favor of a "you are what you play" philosophy, Bethesda is preserving the player’s ties to star signs.
Three prominent nebulae dominate the Skyrim heavens – the thief, the warrior, and the mage. Each of these represents one of the three master skill sets. Each nebula houses six constellations, each of which represents a skill. As in Oblivion, every player starts out with the ability to use all 18 skills – any player can use a two-handed weapon, try alchemy, or cast a destruction spell (provided you find or purchase one). As you use these skills in Skyrim, they will level up and contribute to driving your character's overall level higher.
Every time players rank up their overall level, they can choose a supplemental perk ability for one of the 18 skills. For instance, if you fight most of your battles with a mace, you may want to choose the perk that allows you to ignore armor while using the weapon. As in Fallout 3, several of the perks have their own leveling system as well, allowing you to choose them multiple times. Once you choose a perk, it lights up the corresponding star in the constellation, making it visible when looking up to the heavens while interacting in the world.
“When you glance to the sky after you’ve played the game for a while, what you’re seeing in the sky is different than what somebody else is seeing based on the constellations,” Howard says.
How much is 100% accurate I have no clue.
However going on this I'd say that Birthsigns are now no longer a starting trait effect like before.
Rather you can focus on a parent BS like Thief, then go for the shadow..
If you get all perks related to that constellation then I'd say the last was an uber perk that gave a bonus related to that Birth sign constellation.
However this is speculation on my part.