I think 60 million is certainly possible. If Nintendo decided to raise production in line with this figure, I believe that they would have plenty of ammunition up their sleeves to meet supply. Dont forget that the Wii is still awaiting its first price reduction, and Nintendo have far more room to maneuver in this regard, given the limited technical capabilities of the Wii. On that basis alone I think they could meet the target, yet if you consider the possiblity of a new sku, and the momentum generated by titles such as Wii Fit and SSBB, the sky is the limit really.
On another note, I don't believe the success (or failure) of the HD consoles will have a significant effect on the Wii, given that it offers an experience quite different to that offered by the competition. Indeed, it is the PS3 and the Xbox 360 that compete directly against one another, a significant factor IMO of the Wii's ongoing success.