Starting from the regular as you type setup. If textboxes on the page are being added/removed via Javascript (this covers DHTML and also ASP.NET AJAX type usage, eg TabContainer), 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 TabContainer add a client side event handler for the tab changed event
<script type="text/javascript">
function tabContainer_TabChanged() {
rapidSpell.ayt_setupTextBoxes(true);
}
</script>
...
<asp:TabContainer ID="TabContainer1" runat="server" ActiveTabIndex="0" OnClientActiveTabChanged="tabContainer_TabChanged">
The spell checker will then refresh and underline the errors in the new textboxes.