Dataviz has announced their Documents to Go mobile MS-Office software comes in an iPhone flavor.
The bad news? Right now, only Word documents are supported. A promised Excel worksheet compatibility isn't yet available, but Dataviz offers it as a free upgrade if you buy D2G now.
Still, the promise of creating/editing/synching Word documents in the palm of your hand, all with full formatting retention, is hard to resist. So too is the pricing: US$5.00 for the basic app, and US$10.00 if you want to support Exchange attachments. (Both versions available on iTunes.)