I've asked about this for Freemind 0.9, but apparently there is a freeze on feature requests... Personally, I think that without spell checking all the other features are just gizmo's.
Maybe Simply HTML could incorporate a spell checking feature that could then be used with freemind?
In my naivety, I was wondering if something like the Firefox dictionary add-on might work, or be adaptable? There's bound to be some major flaw in this suggestion, but I'm thinking about whether it is necessary to reinvent the wheel...