Thanks Michael and Jeff for including the chapter on using publish services for ipad export. Thanks to Chris as well for the "super" about ipad3 resolution. I think I got it: the previous suggestion was short-edge at 1536 pixels, which was double the ipad2's short edge resolution of 768 pixels, to allow for zooming. The new ipad is 1536 pixels on the short edge, so double that would be 3072 pixels on the short edge.
One additional thought - the video shows Jeff's resolution as 240ppi. The new ipad is 264ppi, so I'm guessing that would be the optimal setting.
In addition, I find "smart" publish services very handy. For example, I can set it up so my 5 star images automatically get published to the ipad, without even needing to drag the images to the publish service folder.
Lastly, one bug I don't think has been fixed in LR4: I've found that if I publish a group of photos, then rename some, the new names do not get updated in the publish folder when it is re-published. This is a problem because iOS5 reverted to sorting pictures by name, not date. So if I 5 star a picture with filename "00000001", it shows up first in order on the ipad, and even if later renamed to "2012_04_001", so that it should appear after other photos who are named by an earlier date, it still shows up first on the ipad because the filename is not updated when I re-publish the folder. (Hope that makes sense) I've looked around but can't see many discussions of this - just annoying I suppose, but wonder if anyone else has run into this.