Writely's transition to Google's servers takes longer than expected, if we consider that the company was acquired in March. Writely team has announced they've started to send another batch of invitations to those who have submitted their emails. "If all goes well, we should have everyone in before summer ends."
If you don't want to wait until Writely sends the invitation, there are sites that promise to give you one.
It's interesting to note that the famous file that included the list of features of GDrive was found on writely.com. Is that a sign that Writely will be a part of the GDrive project, along with Google Spreadsheets?