I just tested it again.
I added a comment to cell A1 with the text "91 rights issue,29/06/93 split, all other splits correct, splits resolved". In A2 I added a comment with the text "91 rights issue,29/06/93 split, all other splits correct, splits not resolved". I selected A1:A2, chose Format|Conditional Formatting, Formula is =CommentText(A1,"splits resolved"), chose Format Patterns Red and clicked OK twice. A1 had a red fill pattern, A2 had no fill pattern.
So we must be doing something differently. What do you get if you enter:
=CommentText(A1,"splits resolved")
in a cell? I get TRUE (FALSE when applied to A2).