Here's my situation: I have an image field that allows for unlimited values. It's part of my post content type - the first image added is used as a thumbnail for the post, while others are only used if the user chooses to insert them into the post body using Insert.
I need a custom formatter to be used by Insert when inserting images, more precisely a thumbnail with a link to the original file. I know how to make such a formatter for a single-value field and I also know how to make one for a multi-valued field that loops through all of the values and inserts them, but this is somewhere in between. The field itself is multi-value, but Insert is only inserting one of the values (only one image) and needs a formatter for that. I can't seem to define one that would work.
If it is of any help, I used a formatter for a single-value image field a couple of months ago and it was working. Don't know whether it was your update or custom formatters update that broke it for me.
Hope you can point me in the right direction, I'll be asking the Custom Formatters people too in case it's their fault ;).