Currently, you can only have one top level item. However, you have two options. First: you can set ShowStartingNode="False" on your SiteMapDataSource and it will ignore the top level node. Second: If you set the url and title blank on the root node, it won't really render the node although there can be some mild style side effects of doing this.
--
Danny
disclaimer: Information provided is 'as is' and conveys no warranties or guarantees.