Found this on Opera forums few days ago, felt it needed coverage here on neowin too.
http://my.opera.com/Sombria/blog/userjs-orangoo-spell-check
...works as specified and pretty well.
edit: Spell check ver 2 (now works inline instead of popping up a new window)
http://my.opera.com/community/forums/topic.dml?id=180563
edit2: Version 3.5 (if it can be called that, now gives the option to choose between Google's dictionary [requires sign in] and the Orangoo dictionary)
http://my.opera.com/community/forums/topic.dml?id=182282
