Same problem here, no extras (never) enabled.
I removed maemo.org from sources.list.d, and apt-get update works fine. But Application Manager still shows maemo.org. If I update through it, maemo.org is back in sources.list.d, and error is back as well.
Anything changed?
BTW, I cannot install battery-eye, or BatteryGraph anymore...

Update: this behaviour is due to (maybe) archive maintenance. Maybe something is happening... I see a freemantle-1.2 repository in repository.freemantle.org, with date 17th May, and 5.0 is not available anymore (was it?)

Update 2: This morning everything seems fixed, gpg keys on extras have changed.