There is limited support for iOS. It's mostly email and OneNote support. You will not be able to create or edit documents directly, but will need a third party, non-Microsoft, app. You'll want to make sure that app has support for Microsoft's SkyDrive cloud service.
There have been persistent rumors about iOS apps that would be compatible with Office 365, but, in a recent interview Balmer avoided discussing iOS compatibility for the new $99 Office 365 subscription service. So we don't know what is going to happen on that front.
Compatibility with Mac OS X is better but still not great. Only the four primary apps will work on a Mac that accesses Office 365: Word, Excel, Power Point, and Outlook. Those are the big ones, and the ones most people will need; however it's about half as many apps as there are available for Windows.
Most of this was announced today, or was it yesterday? I've lost track.