Past versions included Cupcake (1.5), Donut (1.6), Eclair (2.0, 2.1), Froyo (yogurt, 2.2), Gingerbread (2.3), Honeycomb (3.x), and now Ice Cream Sandwich (4.0?).
To be clear, Jelly Bean has not yet been awarded the name for the post-ICS version of Android. However, it's not as though there are that many treats that start with "J," so Jelly Bean probably has the edge.
It's unclear exactly what will go into Jelly Bean, and even in Ice Cream Sandwich. Since ICS' introduction at Google I/O in May, the belief has been that ICS will unify Android’s phone and tablet codelines (Gingerbread and Honeycomb) into a single codeline, but aside from, that little has been said about the upcoming version.
The source said that some of the big features originally planned for ICS were being moved into Jelly Bean. Despite that, the source said that there are still some fairly major architectural, functional, and design changes in ICS.
Google's Eric Schmidt said recently that ICS would debut in October or November, so this could explain the removal of some features. It was rumored earlier that Google was trying to pull in the ship date of ICS to better match the ship date of the iPhone 5.