Following in Microsoft's footsteps Apple's iWork 09 documents are not backwards compatible. What does this mean? Files saved in any iWork 09 application (Pages, Keynote, Numbers) will not open with iWork 08 unless you choose to save it in iWork 08's format. Apple has published a nice little page explaining iWork 09 and iWork 08 compatibility -- or lack of.
When Microsoft did the same thing creating new document formats (.docx, .xlsx, .pptx) to go along with the release of Office 2007 much of the world was outraged. Businesses were immediately effected because files saved in the default format with Word, Excel, and PowerPoint 07 would not open using previous Microsoft Office versions. People would email out documents only to be notified that the recipients couldn't open them because they didn't have the newest version of Office wasting time, energy, and money. Inconvenient.