From the Office Web Apps Blog:
About a month ago I wrote about the Technical Preview on Windows Live. Since then we’ve seen a lot of interest in Office Web Apps and received numerous requests from people wanting to try them out. To make this possible, we are opening up the Technical Preview and inviting more people to try out the Office Web Apps.
For a limited time, you can sign-up for a Technical Preview account here (if you are already a part of the technical preview, this link will generate an error): http://skydrive.live.com/acceptpreview.aspx/.documents?aobrp=browse.
The features in this expanded Technical Preview are the same as the initial preview; however, during the preview (and beyond) you can expect to see new functionality added over time. Thanks for helping us make Office Web Apps even better!