URL shorteners like is.gd, bit.ly and TinyURL are used worldwide for shortening the longest of links. However, the whole concept of URL shortening is more dangerous than any loophole in software supplying your daily anti-spam needs. Why? Well, domainnames are harder to blacklist if they're hidden behind a service that 301's the request to the Provider of Maliciousness.
The real danger
Of course it sucks when the current anti-spam techniques are no longer sufficient as URL shorteners are so easy to hide behind. It is however not the biggest problem there is with URL shorteners. Why? While you can't link an url shortening service towards executables or archives, you can make a script that automatically submits it for downloading. That script can be shortened by pretty much all services. You'll never know whats behind door number one when you click on it. For example, this url:
Is pretty straight forward. You know you're going to download a ZIP archive, and you know where it comes from. And then this example:
They all redirect to a script that submits a zip archive for downloading. Downloading is all it takes when a Malicious Douchebag is sending stuff around to exploit security issues with browsers and operating systems. This works for any browser on any platform, and is untraceable for current anti-spam software and browsers.
My advise? Anti-spam software should be updated to follow through on any 301 or 302 redirect headers. It should cross reference the given mime type and check for the Content-Disposition header. Besides that any url shortening service that doesn't do this as well, should be put on blacklists along with viagra-is-bloody-cheap-here.com and such.