Starting from the regular as you type setup. If textboxes on the page are being added/removed via Javascript (this covers DHTML and also AJAX type usage), then it is necessary to call a Javascript function in RapidSpell to make it rescan for active textboxes to spell check.
rapidSpell.ayt_setupTextBoxes(true);
On a page using a tabs add a client side event handler for the tab changed event
<script type="text/javascript"> function tabContainer_TabChanged() { rapidSpell.ayt_setupTextBoxes(true); } </script> ...
The spell checker will then refresh and underline the errors in the new textboxes.