I should have given more detail. I understand that you can unprotect the document and spellcheck it, however thie then means that the end-user can also alter other parts of the document, which defeats the purpose of creating a form in the first place. Also, if you unprotect, spellcheck then re-protect the fields are cleared of what you typed.
After looking at the KB article mentioned by AUG4570 it looks like ther is no way around this other than building custom macros into every form. The article claims this only applies to Word97 but I know it also happened in Word95. Maybe this has been fixed in Word2K...
Thanks all.