No one knows for sure whether any current 802.11n routers will be upgradeable to the final version of the standard. That's not something a vendor could promise because it's too early for them to know.
Of course, all 802.11n routers will continue working fine with 802.11g. Some 802.11n routers also support 802.11g MIMO.
Atheros-based solutions are the only 802.11n routers that currently work 802.11n hardware from other companies, such as Intel and Apple. If you have new Dell Centrino notebook or Apple notebook, you probably want a router based on Atheros technology.