Dear carl,
The IFRAME within a splitter panel will be given a name that is equal to the ID of that IFRAME, which can be accessed with the IFrameClientID property of the splitter panel.
E.g. you may add the following code in .ASPX so that a link to Google is created, and on clicking the link, the destination page will be opened in the IFRAME: (IFramePanel is the splitter panel that contains the IFRAME)
<a href="http://www.google.com/" target="<%= IFramePanel.IFrameClientID %>">Google</a>