Google moving data from Writely?
They obviously didn't take down their servers to do hardware or OS upgrades, otherwise nothing would be available -- it has to be related to moving data. The reason they would lock the files and keep them read-only would be to keep everything in sync.
Where would they be moving data? Well, since it's now a Google product -- I am thinking there is a good possibility it is being transferred to Google servers to populate a similar service to Spreadsheets.
Is there any evidence this might be what is happening? Yes there is. The URL for this service will probably not be www.writely.com -- Google likes to make the URL of their services relatively consistent. Because we know it will probably be <something>.google.com, I started digging around to see what I could find.
Currently, writely.google.com doesn't translate to anything, but after several attempts, I found a new DNS record writely.l.google.com that redirects to Google's main page. Usually this is how Google rolls things out -- for example, spreadsheets.google.com's CNAME record points to spreadsheets.l.google.com.
Of course they could be simply performing maintenance to Writely, but I hope not.