Looking at it from MS's perspective, they have 2 parts of their business really, windows and office. It seems that the iPad is damaging windows, so they'll want to fight back against it. An iPad version of office would help the iPad, not hinder it, so don't expect it any time soon
On the other hand, I bet there is an iPad running office somewhere in Redmond. If the iPad proves unstoppable, not having office on it would damage the office business unit. If windows 8 tablets aren't too successful I think they'll release office on the iPad.
Again, I think this underestimates the difficulty of making Office work on a device like the iPad. Apple is unable (I assume) to provide a OSX level version of Pages and Numbers for the iPad. Why would you think that porting an even more complex product like Office would be easier?
It's worth noting that Microsoft already has a simplified office automation product; they've had it for years. It's called Microsoft Works. (Note that Microsoft, unlike Apple, does not name the limited function version of their office automation product the same as the more comprehensive version.) If Microsoft were interested in producing software for the iPad, I'd assume Works rather than Office would be their choice.