The March update of the SharePoint Online Office 365 beta service description has a welcome change; the first 50 external users invited to your site collection are free! This makes collaboration that much easier - from the get-go you can bring in your customers or suppliers in to your site collection at no extra charge to collaborate on documents and projects.
Here's the relevant text lifted from the service description document (emphasis mine):
Sharing Externally: The external sharing capabilities in SharePoint Online enable a company to simply invite external users in to view, share, and collaborate on their sites. A site collection owner can turn on external sharing, and then invite external users to collaborate on sites, lists, and libraries. An external user has access to only the site collection they are invited into. And the best part is every Office 365 customer will get 50 partner access licenses (PAL) free, and customers purchasing our enterprise offerings can purchase additional PALs as needed.