Nokia N900 will not be able to get an update or Maemo 6 as Nokia will not let it happen. They always launch new OS with New Device and its there strategy to do business. If N900 will be able to get Maemo 6 update then the sale for Nokia N920 or N** will drop which will be bad for nokia and they would have already thought about it and that is why I think Maemo 6 will not get an update.

I love this phone and am dying to explore more of it. But I am sure there are many Einstein developers who might make M6 tweak and work on N900 as I do not think there will be any hardware issues.(Hope they do not update the OMAP chip)

Cheers