OK, I have something for you. I actually read RFC 3629 on UTF-8 and found a relevant paragraph:
http://www.rfc-editor.org/rfc/rfc3629.txt
+Implementations of the decoding algorithm above MUST protect against+
+decoding invalid sequences. For instance, a naive implementation may+
+decode the overlong UTF-8 sequence C0 80 into the character U
0000,
+or the surrogate pair ED A1 8C ED BE B4 into U+233B4. Decoding+
+invalid sequences may have security consequences or cause other+
+problems. See Security Considerations (Section 10) below.+
This means (to me) that MacOSX should never produce Cyrillic from a properly working Roman keyboard; it should stop with an error condition. (Check the Console.)
Does the iMac print UTF-8 text files correctly? TextEdit will now allow files to be saved in HTML. In 'System Preferences', 'Open and Save', 'Encoding', select UTF-8. Now write something in English. Does it look jumbled? Save it in HTML and view it. Does it look jumbled?
If so, try a new keyboard (perhaps a USB). If it's still jumbled, the paragraph above guarantees the algorithm in MacOSX is damaged, for it should have crashed TextEdit with an error condition. I should think an OS re-install would be appropriate.
If it prints fine English, I haven't a clue as to the problem--sorry.