The new enclosure support in WP 2.6 accepts an enclosure specified as an <enclosure> element on a submitted post, but doesn't advertise the presence of that enclosure as an <enclosure> element on get requests.
Instead, the enclosures are represented as custom field attributes, which is probably fine, but which doesn't provide a valid way for a basic XMLRPC MetaWeblog? client to get at the existing enclosure value on a given post.
I am not sure whether only the first enclosure should be exhibited as an <enclosure> element, or whether all of them should be. But the <enclosure> element should be used to convey an existing enclosure item to XMLRPC MetaWeblog? clients.
Daniel