From the press release entitled MSN Launches Beta of Soapbox on MSN Video we learn

REDMOND, Wash. — Sept. 18, 2006 — MSN today announced the U.S. beta release of Soapbox on MSN^® Video, a user-uploaded video service that makes it easy for people to express themselves by uploading, discovering and sharing personal videos with the Soapbox community and others around the world. Soapbox will be available on MSN Video and will be deeply integrated throughout Microsoft Corp.’s portfolio of online services, including Windows Live™ Spaces and Windows Live Messenger.

“Soapbox delivers on a critical component of the MSN growth strategy of deepening audience engagement by enabling people to participate in the content experience,” said Rob Bennett, general manager of Entertainment and Video Services for MSN. “By adding a user-uploaded video service, we are rounding out our existing investments in commercially produced and original content on MSN Video.”
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Availability

The beta of Soapbox on MSN Video is available on an invitation-only basis in the U.S. Those interested in participating in the beta can sign up for the waiting list now at http://soapbox.msn.com. Access to the beta will expand over time by enabling existing beta testers to invite a limited number of friends.

I'm on the invite list for the beta but I haven't tried out the site yet. However there is a brief overview of the site at TechCrunch in the post Microsoft SoapBox Just Launched by Mike Arrington. He writes

Om Malik says, via a commenter, that “Soapbox autodetects your browser + platform and streams WM for IE/Windows users, but Flash for Firefox/Windows and Firefox+Safari on Mac.” LiveSide says videos up to 100 MB in size can be uploaded in AVI, ASF, WMV, MOV, MPEG 1/2/4, 3GPP, DV file formats

I'll try out the service myself later this week and see if I can get any invites to share with folks who are interested in giving it a shot once they give us invites.