I didn't mean to start the whole apple vs google debate here..
But let me tell what i find so nice about activities and intents by example of a simple usecase.
I am browsing in my RSS reader and find a good article. Now if i want to share this article, i usually have a "share" option which sends the intent to the OS. Depending on the registered applications it let's me send this article via mail or twitter or sms or even bluetooth (or really whatever app supports this intent). After deciding i want to send this as mail to a friend the mail activity opens up, let's me send it and seamlessly send me back to right where i was (reading the article). This is integrated throughout the whole OS.
Now, if i am reading some rss article in Pulse on my iPad, i have two options: Use the facebooks-share button of Pulse. Ok, but if i want to mail it or the app doesn't offer that at all? I'll have to copy the URL, close pulse, open the mail app, write the mail, close the mail app, open up pulse again.
Another example is: Google places offers me a restaurant i want to visit. Now depending on the apps i have installed i can use google maps, some other navigation software or the public transport app i have to show me the way to get there. It makes the user experience quite seamless, despite the applications not "knowing" of each other. I don't have to write down the adress, open up some other app, enter the adress again.
It's also the way to replace core functionalities of the UI. Want another home application? When pressing the home button on the device it's firing an intent and this can be handled by some home app of your choice. Same for the search button.
The nice thing about intents is that it's very flexible. Depending on the apps i have installed it offers me interaction between those apps.
About JIT: If you want to believe Wikipedia, JIT is no JVM originated feature but was invented somewhere in the 60s.
I find it quite hard to imagine how the Dalvik JIT could be a real rip-off of the JVM JIT, as those are based on quite different architectures. The only thing in common is that they both use the same syntax.
About Open Source: I may be corrected here but it's perfectly possible to have a working phone by taking the sourcecode from android. It will have a running kernel, a working UI, i can install apps and live with it. What would be missing would be gmail or the android market. But there are other markets and other mail applications.
Could you please point me to where i can find a compilable open source version of iOS?