The good thing about owning a Nexus (One or S, and soon Prime), is that you get update almost immediately after it is released by Google.
This is because the Nexus range runs Google's Android software. The difference between the Android OS found in other phones and the one in the Nexus range is that the latter has 'pure' Android -- without any bells and whistles, while other Android phones have customisations by the companies.
This means that because of the extras packed in by other mobile companies in their Android versions, the updates released by Google for Android will have to be modified suitably so that they don't affect the customisations. Because Nexus phones have no extra bloatware or customisations, any update released by Google can straightaway be installed.
Even for Nexus phones, the updates may take a week or two to be rolled out for all phones. But whenever it is available, you can just download and install them. You get a notification that a 'system update is available'. All you have to do is to install it.
The same update can take even months to hit other Android phones. And some phones may just not get the update.
