TrackBack is now enabled in HubMed searches. You can see forward links by clicking on a'References' link. You can use this feature to notify users of HubMed when you've posted commentary about a specific paper elsewhere, or if it's included in a list of references.
A TrackBack-enabled Movable-Type bookmarklet will pick up the pingable URL from any abstract display page (reached by clicking on an 'Abstract' link). If you want to automate the pinging process, the TrackBack URL is "http://www.pmbrowser.info/hubtb.cgi/" followed by the PubMed ID number, eg http://www.pmbrowser.info/hubtb.cgi/12529850
Sam Ruby has written a nice clear explanation of the benefits of external comments and trackback linking.