If you run Windows Live Writer Beta2, and your system’s regional settings is not set to en-us, then the spell checker probably won’t work.
Quite funny, since you’d probably need that most if you’re NOT natively speaking English.
Anyway, it seams that I’m a complete idiot as I haven’t noticed a fix that is apparently out there for some time now.
So, if youwant your spell checker back, go tothat blog post.