Yes, mine is back. I think they're now working on individual case-by-case accounts to restore the application. It's strictly a Kobo issue, not the PlayBook, to do with their accounts. If you're desperate and you don't have any paid books, you can sign up for another, new account and the application should work.
Did anybody whose Kobo is working again have to do anything (contact the company, complaint, etc.) to get it back working?
Mine STILL isn't working and hasn't been.
I did send them an email. i got an automated response. i kept monitoring the forum and when several posters said theirs was working I tried mine and it worked. There is a thread in the games and app forum that you may want to investigate.
Mine came back up yesterday. I had read another thread about it being up for some. When I originally opened Kobo there was nothing, but once I rebooted it was fine.