Update (03/27/14): Microsoft PowerPoint, Word and Excel are now available for the iPad. Each app is free to download and allows you to read Office documents, but you will need an Office 365 subscription to create and edit.
Microsoft announced on October 16, 2013 that they're working on a touch-optimized native version of their signature Office suite for the iPad, but they did not give an official release date. There are also a few existing Office Web Apps available for the iPad.
Update: Microsoft released a Remote Desktop app for iOS devices on October 18, 2013.