Thanks, Nauman... I've got something working now and took some from you to fix the problem of the ParentNode being Nothing.
However, I was working in VB.... and it seems that you can't use operators to just compare the nodes as you can in C#, so I had to compare the Url.
NodeWalker = SiteMap.CurrentNode
(NodeWalker.ParentNode.Url <> SiteMap.RootNode.Url))
NodeWalker = NodeWalker.ParentNode
lblTopLevel.Text = NodeWalker.Title