There's a big problem with Skype on the Mac: and no, it's not its ugly UI (although that is a big problem). As noted by Mashable's (and former TUAW blogger) Christina Warren, the latest version of Skype for Mac has an unpatched security flaw that that allows a person to gain remote access to another's machine simply by sending a Skype message. The flaw was discovered last month thanks to the work of researcher Gordon Maddern from the firm Pure Hacking.
Maddern contacted Skype, who was reportedly already aware of the vulnerability and working on a fix. They then issued a hot