Did you turn on spell check for the subject line?
I've also found F7 seems to spell check the subject line.
"Pressing F7 to initiate a spell check will also check the subject."
But no where did I find an indication it used another dictionary.
Outlook 2010 uses different dictionaries to spell check the body of the email and the subject line. Can easily edit the regular File/Mail/Spelling and Autocorrect dictionary but I cannot find the one used for the subject line.
Thanks in advance,