Actually, Android is released under an Apache style open source license. Anyone can build a device around it. There are just a few nice google provided applications for Android that aren't released under the same license and have to negotiated for separately. There are quite a few community provided android releases for existing devices which have not received official upgrades to newer versions, or which (not legally) add back the google apps.

A lot of Android phones are getting officially provided upgrades to newer versions of the platform. There are a few which aren't, such as the G1, which can't be upgraded for technical reasons (not enough flash storage to hold the rom image, etc.).

The standard that is emerging for both Android and Iphone platforms is of a fairly long term support window on the order of 5 years. Applications are forwards compatible over that time frame. New OS releases are back ported to older hardware while introducing new functionality and API exposure for hardware features found in newer phones.

Nokia is still thinking in terms of a 1-off device mentality and with the N900 the window of support compared to previous devices has shrunken remarkably, with few seasoned users expecting any substantive development / fixes for the platform past a year from initial release. If we are lucky, we will have some degree of compatibility with QT applications released for Meego, but there are tons of core functionality issues with the N900 that will never be fixed/addressed by Nokia & are unlikely to be addressed by community releases or impossible to do so due to closed source components.