Mozilla-based browsers such as Firefox and SeaMonkey applied spell checking to textareas as well as iframes in designmode from the very beginning.
As a simple test case you can go to http://www.kevinroth.com/rte/demo.htm and type some random letters. No red underlines will appear as they would in normal textareas.
This works correctly for me in both Safari 3.1.2 and with the latest nightly build on Mac and Windows. Please be sure that you have not unchecked the "Check Spelling While Typing" item in the "Spelling and Grammar" context menu item.
Unfortunately I do not have a Mac to test Safari and I had to rely on a bug report from one of my users who reported a functioning spell checking in textareas, but not in editmode iframes, suggesting that the cause of the problem is not that "Check Spelling While Typing" setting.
I will ask him for his Safari version number and will report back here.
I have also replicated the problem myself in Google Chrome (which uses an older Webkit version and does not have user accessible options to turn spell checking on or off).
Does it also work when you use the wikEd gadget for editing Wikipedia (register a Wikipedia account, go to Preferences > Gadgets and check wikEd, and edit an article).
If you have Windows you can easily test Safari yourself.
Yes, I have just figured that out and it actually works under Safari 3.1.2 under Windows XP when tested with the rich text editor wikEd.
The user insists that the problem is real and that the spell checker is enabled. His user agent is:
Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10_5_5; en-us) AppleWebKit/525.18 (KHTML, like Gecko) Version/3.1.2 Safari/525.20.1.
(The bug discussions can be found here:
http://en.wikipedia.org/wiki/User_talk:Maury_Markowitz#Spell_checker_under_wikEd_-_need_browser_version and http://en.wikipedia.org/wiki/User_talk:Cacycle/wikEd#Interferes_with_spell_checkers.3F)