According to the DataViz, the developer, Documents To Go delivers support for Microsoft Word, Excel & PowerPoint, PDF, Apple iWork and other files. It includes a desktop application (Win and Mac) to provide 2-way file synchronization.
It is supposed to be the only solution currently available that supports Microsoft Word 2007 editing on the iPhone. Excel editing will be offered as a free upgrade when it becomes available. Learn more about the app here, which is currently discounted at $4.99, at the App Store.
A version with support for attachments is also available for Microsoft Exchange e-mail users who need to send, receive and edit attachments on the iPhone. This version goes for $9.99 and is available here.