nocebo - The mods removed the link, as details of the exploit were in it. My sincerest apologies for that.
Basically, there's a way to exploit the user control panel that will allow someone to transfer your domain onto their own account, which would mean you'd lose control over it. The guy who discovered it found out about it and let the domain host know months ago, and they did nothing about it. He posted it to his blog ten days ago.
I would move your domains to another registrar ASAP.
Although it does make me worry about other stuff, the exploit has been fixed.
Dynadot claims they could never duplicate the exploit: "This supposed exploit has been circulating around the web for a few weeks now. We were not able to reproduce it. We have received no complaints of domains being stolen this entire time."