[Obligatory Warning: The information below is merely a speculative issue. I am not suggesting its use on any site. It is, however, something to watch for.]
Now back to news aggregators. When you submit a link to those sites, you can chose a title, add the URL, and then it is instantly visible to many users of that site. The point of these sites is that news-worthy links and important information gets "voted" to the top and that meaningless data is sent to the bottom. However, in order to be sent to the bottom, some people will have to click and rate the submitted site. This is where we bring in the malicious URL from before. If the URL with injection code is passed through a single or multiple URL shorteners, there is no way for the user of the aggregation site to know what is behind the link.
The entire point of this concept is to distribute a hacking attack. If an attacker is trying to attack a site through a form of injection, he or she can simply craft a ton of malicious links, submit them to these aggregation services from behind a proxy, and have some innocent user click them for him or her. It's effectively having others hack a site for them because when the site checks its logs, it'll see multiple IP addresses from users of the aggregation service.
Is this a big concern? Not really. But it's an interesting possibility that site admins and security professionals need to watch for on their own sites.