What it is, mostly is Software as a Service (SaaS). In this case the software is a shared Web site editor. Sure, you can pour your other Google apps into it, and pretty sure you've got teams sharing your Google data, but that's not Sharepoint.
Sharepoint, as our own Matt Asay has warned us with all the subtlety of a Republican operative using Barack Obama's middle name, is a glue aimed at tieing you to a Microsoft file format in order to share data across an enterprise.
It's quite popular, and as its popularity grows it becomes harder-and-harder for any enterprise to leave Microsoft's clutches and stop paying it license fees.
If Google Sites is anything like that then it is not, by definition, open source. No one has described to me the file format of Google Sites. No one has shown me the license for using it.
Matt's view must be taken with some salt, since his employer, Alfresco, is "a company that provides an open-source alternative to SharePoint." That disclaimer is at the bottom of each post he does on SharePoint.
But in looking at Google Sites in terms of Sharepoint, Matt's questions are the important ones. Does using Sites lock down your data so you can't stop? Is using Sites really the same, in any way, as using Sharepoint?
My guess is the answer to those questions are no, and no. My guess is the output of Sites is plain HTML, so its format upgrade path is toward HTML 5. Use of Google is based on a Terms of Service agreement, not a software license.
Now, might you and yours become so dependent on using Sites that you dump your entire corporate data stream in it and kiss Sharepoint goodbye? My guess is the answer to that is no, if you're big enough to be a good Sharepoint prospect.
But you might grow. So can Google Sites scale with that growth? We won't know the answer to that for some time.
So this isn't a potato-potahto question. It's more like a potato-banana question. The question of Sites or Sharepoint really depends on where you think the growth of computing will be, on your hardware or in the cloud's?
That's the real trillion-dollar question. That has always been the question. Sites doesn't answer it. I don't think it's supposed to.