You have always been able to work on documents simultaneously in Office Web Apps and Word on the desktop with SharePoint Online in Office 365, the only difference was that it would do a paragraph lock. This differed from the Google Docs approach which showed you real time editing of everyone. Basically you could follow every change they made to a document. Microsoft’s research indicated that was distracting to authors (and it is for me I know) so that is why they initially did it the way they did.
However, it seems that having the ability to see people individually editing every item in a document has won out, so Microsoft has now updated it’s Office Web Apps in SkyDrive and Office 365 to include this feature. You’ll get a better idea of what it looks like in this video:
This now makes the Office 365 editing experience more like that in Google Docs and removes a perceived difference between the products (and to be honest what appeared like a limitation in Office 365 even though it wasn’t). That is great news for Office 365 and makes the sell to customers who want a Google Apps like web editing experience much easier.
It will be interesting to see what Google does about this as puts the ball back in their court to come up with another differentiator. In fact, I think it puts the pressure squarely on them to come up with something as rich as what Office Web Apps is. Google Apps is looking pretty basic compared to Office Web Apps these days I reckon.