In script block or code behind:

Public contentUrl as string = "main.aspx"

Private Sub Page_Load(<args not included)
If WhateverTest = True Then
contentUrl = "here.aspx"
End If

DataBind()

End Sub
In the html:

<frame src="<%# contentUrl %>" id="MainContent" name="MainContent" frameborder=no />

It works better than the Javascript redirect, which I was using before. It cause the default page to flash quickly before the redirect. It also caused the default page to perform all of its processing for nothing.

Thanks,
Sam