Keep your eyes peeled for a directory of third party iGoogle themes coming soon. As you can see from this screenshot, Google was caught toying with the attribution feature for them. Likely, the XML file for third party themes will have a node allowing the creator to specify an "attribution image" to be displayed in the footer of the iGoogle page.
Currently, themes are only hosted on Google's servers, but if these custom themes are similar to Google Gadgets, an XML specification will likely be hosted on an external site, then cached by Google to keep them loading fast. If you have any information on this upcoming feature, post a comment or send me a message.