The iPhone famously doesn't allow applications to run in the background, except for some built-in apps like Mail, iPod or Clock. Apple says that this restriction saves battery power and makes the iPhone easier to use. I did agree with that, to some degree, but after using Android for a couple of weeks, things look a little different.
So is Apple right and background applications deteriorate the smartphone experience?
First, background applications can obviously do things that iPhone apps cannot. Remember The Milk, for example, can reliably notify you about due tasks on Android. The iPhone version uses push notifications, which don't work without a network connection, means you cannot rely on being alerted in time. DoggCatcher and Slacker.com can download podcasts and music during the night - the iPhone relies on desktop iTunes to do that. The consequence is that you can operate an Android phone completely without a computer, but not an iPhone, at least if you want to use it as a music player.
On the downside, you have to think a little bit more about how to set up apps in Android. On the iPhone, it's not a problem to install dozens of Twitter apps, because only one can be active at a time. On Android they would all happily sync themselves with Twitter at the same time and suck your battery dry for nothing. For apps that download bigger files, such as DoggCatcher, you need to make sure that it's set to WiFi and/or connected to power only to protect battery life. Every app has its own settings and every app has its own bugs, so don't take it for granted that those settings always work as expected.
Another issue is which app controls the headset button. In my experience, it's random: so pressing the button while listening to Slacker may indeed stop the music or it may start DoggCatcher to play at the same time, which is never what you want. The latter has a setting to always take control over the headset button, but then it doesn't work anymore for the phone, which should have priority in all cases. I hope that Android addresses this in a future release.
I don't see any issues with the battery, but the Nexus One does not get a 3G signal on AT&T so that makes it difficult to compare battery life with the iPhone. Interestingly, though, I don't miss 3G a bit.