Please please do yourself the favour of clicking through the article to this blog post explaining the matter in more detail.

Let's see if I can give you the gist of it:

That Google is using the kernel is great. Forks are not unusual. That's not a problem. Google has been doing certain changes, which mean that Android partners must target their products (hardware drivers, Android customisations, physical devices) directly to Google's branch; neither reading nor writing to the kernel.org tree would make any sense. Also fine (if a bit inconvenient). Now, the problem: Google does not merge their tree into the mainline. This means that even if you (being an Android contributor) wanted to, you can't contribute your software to the general linux cause because "you can't get there from here". That some still insist to get their software into both the linux kernel and Android is a testament to their dedication, but currently they're being "rewarded" by having to do double work. It would be better if Google invested the time once and for all to overcome that, but they show no signs of wanting to do so.
 
Please please do yourself the favour of clicking through the article to this blog post explaining the matter in more detail.

Let's see if I can give you the gist of it:

That Google is using the kernel is great. Forks are not unusual. That's not a problem. Google has been doing certain changes, which mean that Android partners must target their products (hardware drivers, Android customisations, physical devices) directly to Google's branch; neither reading nor writing to the kernel.org tree would make any sense. Also fine (if a bit inconvenient). Now, the problem: Google does not merge their tree into the mainline. This means that even if you (being an Android contributor) wanted to, you can't contribute your software to the general linux cause because "you can't get there from here". That some still insist to get their software into both the linux kernel and Android is a testament to their dedication, but currently they're being "rewarded" by having to do double work. It would be better if Google invested the time once and for all to overcome that, but they show no signs of wanting to do so.

Very well summed up. This is why I'm leaning more toward Maemo recently, although I don't like how Nokia handles updates on their phones....