They were obviously in development at the same time for the most part.
Apparently not. They started developing the iPad then released the iPhone instead. Allegedly there are people on the Apple campus who had seen an iPad 2 before the iPad was released.
As for the iPad getting thinner, rumours of it having tapered edges and a flat back like the iPod Touch. This would mean it could rest of flat surfaces.
Retina - Not a chance in hell. The resolution would be way too big something stupid like 2200x1500.
Like Raymond Lin, I don't expect anything to be announced or released until April time. This would be a year since the release of the last iPad. I expect them to announce and release in a very small timescale too. (Ready for sale within the week of announcement).
They do this on purpose to help clear stock of existing product and maximise the profits on this existing stock. If they announced a new iPad but told you it would be 90 days before you could buy it, no-one would buy the remaining stock of the existing iPad. By announcing the product when they are ready to ship it, they can slowly reduce the amount of stock of old iPads being sent to stores until they have practically nil (while secretly sending stock of the new iPads to the stores so they can sell quickly after announcement). This means they don't get left with a huge stock of the old iPad unsold.
Also, to back this up, on established product lines they have historically announced and released almost simultaneously. It has only really been totally new products (Original iPhone, original iPad) which had wait times of longer than a month before available for sale, Or where there has been a manufacturing problem which has caused the delay.