Why would Nokia upgrade consumers to meego when they want them to buy their latest device? For hackers, upgrade to meego will be trivial (one parameter different on the flash command). And completely at our own risk. ;)
As for openness, I will have only two criteria : can I still get root and is most of the source available. Given that these are both true for N900 I'm hoping that it will remain true for future meego devices from Nokia.