I beg to differ. It's released tomorrow in the USA. many other countries have already gotten them (for example some people in the UK and some in Australia have gotten them). Macrumors is not just USA, but the entire world.
Umm the first poster after your OP pretty much summed it up, although LTE / 4G wasn't mentioned which is a pretty huge update too.
In terms of disadvantage, there is a weight difference (hardly noticeable, but still there) as well as a tiny bit thicker b/c of the battery size. I also hear Retina apps are going to be bigger, so with 16GB of storage, you will need to manage space more efficiently (and this relates to all products i think). Of course with the iPad 2 prices dropping extremely low (I believe some people were able to get $299 + tax for an iPad 2 (new) at a few B&M stores (spread b/c of price matching)
That pretty much sums it up. If you are interested in many more opinions, I suggest Googleing some reviews. One huge thing I have noticed is that many reviewers are saying that upgrading form iPad 2 --> iPad 3 is a fuzzy line. It's more like a "If you got the extra $, then sure why not" type of thing. It's a definite upgrade opinion from a 1 to a 3, but the 2 and the 3 are pretty similar except for the things listed in this thread (retina, quadcore GPU, 4G/LTE, battery, etc.
The biggest advantage is the retina display (3.1 million pixels on the 9.7 inch screen! Can't wait to get mine!!