We'll i'm sure the game is quite playable right now and it's not beta. Todd just demoed 45 minutes of the game to 105 members of the press.
"Playable" means that the game in it's entirety, is stable. For Demos, they essentially quarantine off a segment of the game they want to show, run the same debugging processes through it to essentially bring it to a beta version, or perhaps a retail-worthy version. Anything we see in E3, is 99% likely to be indicative of the final product's quality. This press event wasn't exactly the Superbowl of PR. In E3, they're going to highlight their best players (Features,segments of the game) and thrown them in to show off. To do anything less would be pretty stupid.
I guess Skyrim will be alpha until they are done with it then
they are adding new quests all the time. But if you don't count adding quests into the game.. Then it might reach beta state at early summer.
Well, I specifically said "Feature locked" individual quests are not features. There is virtually no way to break the foundation of the game by adding in a mundane quest. Quests would fall in the "Content" category. A better example might be, Polearms which, as a weapon type, is a feature, but the individual spears, are content. Programming in the mechanics of polearms could break combat altogether in many ways, but actually adding in a spear object, will not.