We just came across this issue as well. We are on 10.0 SP4. SDE in SQL.
Join 2 feature classes that you could edit before, once joined, Field Calculator is grayed out and despite being able to start an edit session, when you type in a field, we can't edit.
If I export the info from one of the feature classes as a table, and join that to the first feature class, I can edit and use field calculator.
Not sure when this went away, but the help says I should still be able to edit the originating feature class fields in the table, just not the joined feature class fields.
Help!