Now this: Mac malware is now circulating across the Net via pirated copies of Apple’s new iWorks software.

The “iServices.a” Trojan hitchhikes on iWork ’09′s installer. The installer for the Trojan horse is launched as soon as a user begins the installation of iWork, following the installer’s request of an administrator password.

Once the Mac is infected, the Trojan phones home to let daddy know it arrived safely and is awaiting further instructions. Since then, the same bit of viral nastiness — dubbed OSX.Trojan.iServices.B — has begun showing up in pirated copies of Adobe Photoshop CS4, according to Macworld.

Glass-half-full types can view this as further proof of the Mac’s success; it’s now installed in sufficient numbers to be worth the attention of botnet herders. Also: That room full of manure is sure to have a pony inside.

The computer worm responsible for the biggest attack in years has infected at least 1 out of every 16 PCs worldwide, a security company said Wednesday, and may have managed to compromise as many as nearly 1 in 3.

According to Panda Security, almost 6% of the Windows systems scanned with its antivirus technology were found to be infected with “Downadup,” a worm that began aggressive attacks just over a week ago. Panda was one of the first security firms to ring the alarm over Downadup, when it raised its security threat level Jan. 12 as reports of attacks mounted.

Using data from antivirus scans performed by its consumer-grade security software, and by a free online scanning tool it makes available on its Web site, Panda found 111,379 PCs infected with the worm out of a pool of 2 million machines.