Started feeding citation data into HubMed today, so the usual Trackback system for comments isn't available for a while.
PubMed Central/Biomed Central were able to provide about 100 of their full-text XML articles, including citation data. With nicely structured data like that, it was fairly simple to start stringing articles together with the standalone trackback module. See this link for an example: click the Cosmos link to see all the references for that paper; follow the links to the referenced abstracts; click their Cosmos links to work your way back again.
The next stage now is to integrate the data from all Biomed Central papers, then work on to the other publishers. If anyone has email addresses for contacts inside commercial publishers, please get in touch.