This has nothing to do with Tweetdeck other than someone using the name. That's like saying someone who tweets about a "Critical Microsoft patch!" has hacked Windows.
No one hacked TweetDeck (the Trojan isn't even an AIR application), and digital signing can only be done if the hackers get their hands on the original certificate. Since neither of these happened, 90% of this article is completely incorrect. Please stop blaming Adobe for your ignorance.

Adobe's Flash is the most successful resource-consuming trojan set loose on Internet, and a lot of people pay for it.

The program that people are downloading is called "tweetdeck-08302010-update.exe"(*). It's just a completely ordinary executable(**) that pretends to be something to do with tweetdeck but isn't. Since it isn't really an Air application, Adobe's Air installer doesn't even come into the picture.

Also, according to the Adobe devnet page you linked to(***), self-signed certs don't work, unless somehow you managed to get the user to install your self-generated CA cert into their root cert store first: to quote,

"Note: There's also a way for a developer to self-sign an Adobe AIR application so they can test it, but when the AIR runtime tries installing the application, it presents the user with a big old UNKOWN publisher warning, see Figure 1(****). (Unless, of course, your user has installed your self-signed certificate on his or her machine.)"

It's surely incorrect to describe a self-certified cert as "trusted" when it pops up a huge great warning like that; and as I pointed out, it's not Adobe Air's fault that completely unrelated applications can run and install themselves on the machine.

Update/clarification needed?

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(*) - http://support.tweetdeck.com/entries/249941-do-not-download-fake-tweetdeck-update-appearing-on-twitter
(**) - http://www.virustotal.com/file-scan/report.html?id=73a57edb2e301b0bff4c5f301e160aa433f8abae737bf0cd4dc1e4c44e1a05dd-1283261376
(***) - http://www.adobe.com/devnet/air/articles/signing_air_applications_print.html
(****) - http://www.adobe.com/devnet/air/articles/signing_air_applications/fig01.jpg