According to PC world,
Microsoft hosted office web apps will be free for users with Live accounts
Customer hosted Office Web apps will be free for enterprise customers with SA (Software Assurance)
This web version will definitely have limited functionality, compared to the desktop version. But I am sure that still it will be more attractive than Google apps. The best part is that with this move SharePoint 2010 becomes even more powerful with online document creation and editing facilities. (check the screen shot here)
Let us see what the Google will do in return.
(Read the source article here.)