Apparently there are to be New Cover sizes for ebooks/ibooks after 15th. June 2012.
The sizes are to be 2,500 pixels high and no less than 1,400 pixels wide. Or thereabouts.
It is to do with new packages and updates for Readers here shortly.
If you do have ebooks/ibooks published prior to that date you do not have to change them.
If you have book covers you have not yet published then you will have to make new ones to the above sizes.
If your cover is photographic you can't just enlarge to the required size. If you do it will cause pixel blurring.