I just got an email from Writely:
Dear username,
In a few days, we will update your Writely account to use your *****@gmail.com Google Account registration settings.
This means you’ll need to use your Google Account password when signing in, not your Writely password (if they’re different).
If you’ve forgotten your Google Account password, just go to https://www.google.com/accounts/ForgotPasswdThanks!
Google Writely Team
This is just the start of whole Google GDrive. Google has already released a spreadsheet applicaton, calendar, and some others. The addition of Writley to the Google Account is great for everybody.