If you use aspell you'll get a lot more from the spell-checker than if you use ispell from a pure OACS functionality POV, so I strongly recommend that one ...
With aspell, you can now choose if you want to work with dialects or just the language(s) proper (the aspell config file on the os will then decide which dialect to use, if there are more than one dictionary available). The parameter is called "SpellcheckDialectP".
When the SpellcheckFormWidgets parameter is set to "textarea 1 text 0", for example, it results in a pull-down with the _os_ default dictionary pre-selected for all textarea widgets, and a pull-down with "No, thanks" pre-selected for all text widgets (unless the "nospell" flag is present in ad_form's element declaration, of course). I'm hoping that this behaviour makes sense.
If you only have ispell installed only two choices will be offered in the pull-down; Spellcheck: "No, thanks" and "Yes, please" ... ispell's default dictionary is used if you say yes.
With neither aspell nor ispell installed, spellchecking is but a memory, sniff (the pull-down is removed).
Any help with testing this piece of the form builder is greatly appreciated, of course. It should be available on the test servers in the morning, or evening, or ...