There are two possibilities.
#1 apple is confident that current iPad apps display on the new device decently without requiring an update. In this case the iPad could very well launch right after the announcement.
#2 current iPad apps do not look as good as when they were displayed on the iPad 2. In this case, apple would give at least one week after the announcement before launching the device, just to give developers time to submit enhanced versions of their apps so that they are ready when the device is released.
Yes, but they gave developers advanced notice to make changes. While they don't force developers to update their apps, the ones that do can spend weeks just waiting for the updates to be reviewed by apple before it is approved for the app store.I'm pretty sure the iPad 3 will launch this Fri in the US. When the iPhone 4 was released, it took weeks for many apps to get updated, some even took months. They still worked but were just not optimised for the retina display. I don't think this will have any bearing on the actual release date.
Either way, we will be seeing a huge flood of app updates in the coming weeks. Get ready to see a bunch of app updates every time you open the app store.
I think more about how much extra HD space will be used to upgrade the images. I really hope they (Apple) give a option to buy something larger than 64GB. An HD movie can be over 5GB each. An HD game must be vast.