This week we've encountered a cross-platform worm that's capable (at least theoretically) of spreading from a PC to a mobile device and back. To be more specific, the "Mobler" worm moves between Symbian and Windows platforms. Although it's quite nasty on the Windows side, it doesn't cause much harm on the Symbian device. It just copies itself to the memory card and tries to trick the user into infecting his PC.
Technically there isn't any automatic spreading mechanism for Mobler to copy itself from one platform to another. It just creates a Symbian installation package that inserts a Windows executable on the mobile device's memory card. This executable is visible as a system folder in Windows Explorer - so it's possible for the user to accidentally open it and infect their PC while browsing the memory card's files.
Mobler poses no immediate risk to mobile device users in its present form. However, it's possible that virus writers might use it as a basis for more malicious malware. But then again, that could be said of previous cross-platform viruses and thus far a heavy hitter has failed to materialise.