It's another one of those fragmentation problems for developers. If you want to make your app work across all versions of Android so you don't have to maintain multiple versions, you need to code for the lowest common denominator or try running code only for ICS if needed. It's pretty ugly.
No, because the Android developer guidelines deal exactly with that particular issue, how to code an application properly so that it looks at home on 2.x AND on 4.x. Like others said, it'll take a while for app developers to update (if they bother at all, but those who won't bother will see their apps diminish in popularity surely as more and more phones are released with ICS). (as far as I know, I haven't delved deeply in Android app development but I did skim over the document in question on the Android developers' blog/site)
Keep in mind at the moment, ICS is only available on two phone models, which aren't available everywhere on every carrier (yet). But I did notice a lot of "what's changed" in the Market app descriptions mention "ICS compatibility"...
I have a GB phone and a Froyo phone (both Sharp-made Japanese models) and an ICS phone (Nexus S).
Let me tell you the whole fragmentation argument is overhyped, because all of my apps work on all 3 phones, and they work very well. Some of them do appear differently on my Nexus S, because they have been updated to work with ICS's new UI/UX paradigms. Yet the apps look at home on Froyo and Gingerbread too... And often these ICS-optimized apps will even show the top-left app icon that acts as a go back button, as well as the search icon on top... Making apps way more consistent accross those 3 different versions of Android.
Yeah if you take into account all the apps, some that look ICS-ized and some that don't, you can probably cry fragmentation all you want, then again not even iOS has 100% consistency between apps, not OS X, not Windows... Thing is, Google is slowly steering developers and OEMs in the right direction without betraying their own ideals of "Android is open, let's see what people make out of it, and we can even take back good ideas into stock Android; everyone benefits".
(as an example of that last sentence.. the text selection/copy/paste that came up in Gingerbread - or was it Froyo? - didn't originate from Google... HTC Sense phones used that exact same system much before, and I'm guessing Google liked how it worked and implemented it, thus freeing up HTC from having to code it so now everyone can come up with more, better ideas! I like it, despite the chaos)
soooo... overblown argument.
Patrix.