This is interesting but is there any commitment to support it long term? I remember when Silverlight was supposed to be the savior that brought .NET to Mac and Linux, but that didn't last more than 5 years. Would be nice for MS to say they guarantee support for 10, 15, etc. years.
One thing that's nice is that it's now open source, so as long as it remains open source, it will likely remain cross platform. Contributers that are on non-windows platforms will likely notice when something breaks on their platform, and get it fixed.
If you're not porting a .net application to MAC or Linux, I would avoid this for quite a while still. We don't know where this is going long term.