Latest M900 is 160.WWE
you can get the official firmware upgrades (although no patch notes are available) from:
-> service and support
-> Smart handheld
-> M Series
-> M900 (Select your model)
They seem to classify them as "Smart Handhelds" and so its not immediatley obvious where they are.
On the good and bad side, the M900 already has a rom uprgade before its even launched. I supose its too much to hope for the perfect O/S on a launch model and the swift release of this ROM does bode well for an aggresive support model from Acer.... fingers crossed!
Happy Flashing ....