Until now. Yesterday on the Webmaster blog, Google announced a new cross-domain canonical URL element. Now, if you have the same content on two domains, you can indicate to search engines which one is the preferred URL.
The element uses the same syntax as the prior version, but now you can indicate the canonical URL is on a different domain:
<link rel="canonical" href="http://www.NEWexample.com/product.php?item=swedish-fish" />
As part of the announcement, of course, they review other ways to handle cross-domain duplicate content, including 301 redirects. However, if you can’t use server-side redirects, the cross-domain canonical URL element can help search engines find the new content and possibly use that new URL in search results.
In the questions sections of the post, they note that you can’t use the cross-domain canonical URL element to just redirect search engines to a new root site (it’s for 1:1 mapping of substantially similar pages), and that pages with redirected cross-domain elements should not use meta noindex.