Someone else reacted that Office Live does work on the iPad (I never tested it).
Anyhow - as web apps are not what ticks (at the moment) on the mobile devices (Apps are) - I was referring to an actual App version of Office.
Given the numerous reported problems that small-scale developers have had with App Store policies, up to and including pulling their software product from the store completely, I wouldn't want to invest time and money in building even a simple iWhatever app. Why on earth would Microsoft spend a fortune building a highly complex app when its availability would be at the mercy of a direct competitor?
Someone should really send a copy of that Ballmer "Developers! Developers! Developers!" presentation to the guys at Apple. They aren't the only guys in town with funky devices any more, and despite all the hype, they are not some sort of unassailable market leader either. If businesses find it too hard to develop and sell software on Apple kit, whether it's because of the danger of rejection from App Store, or not having Flash support, or whatever is going on with Java, or something else tomorrow, then platforms with better developer relations are going to pull ahead, and this is a fast-moving industry.
Microsoft has a history of always developing for their own (just as well complicating) platform, so does Apple. From both perspectives I would say: why not make something (good) available to as many users as possible. Apple built in Exchange support in Mac OS X (and iOS). That is a product from their direct competitor...