So I screwed up. In my zeal to make sure my blogs are fully spell-checked, I added a misspelt word to my Windows Live Writer user dictionary.
There are no easy way (at least, as far as I could tell) to remove it. I ended up doing the following:
Open in notepad:
%APPDATA%\Windows Live Writer\Dictionaries\userdic.tlx
Fix the spelling, save the text file back.
Restart Windows Live Writer
I hope this helps someone out there.