Many thanks to a friend who pointed out a way to rescue the situation I blogged about yesterday.
As far as I can tell, the mobile phone requirement is still mandatory, but you can now postpone it. Whether you can go on postponing it indefinitely I'm not sure, but it will at least give you access to your own Yahoo groups again.
Here's what to do if you find yourself in the same situation:
1. Log out of Yahoo (not just a group, but Yahoo itself).
2. Log back in. At the screen where it asks for your mobile/cell phone number, click 'remind me later'.
3. This should take you back into Yahoo, but with one notable improvement - you now have access to your groups again.
I'm thinking Yahoo may have responded to a welter of criticism, because the log-in screen I saw today was subtly different from the one I saw when I tried to log in last weekend. No matter. I can now access my groups again, and even if it's for a limited period I can at least warn the moderators that I might suddenly vanish in a puff of smoke!