Mobile Spy, a new application from Retina-X Studios, monitors calls and text messages and reports them back to a central server.
When installed on a Windows Mobile device, Mobile Spy allows parents to keep watch on their child's phone, or enables employers to enforce an acceptable use policy on their staff - though the legality of doing so will depend where in the world you are.
A similar application called FlexiSpy has been around for Symbian handsets for the last year, offering much the same functionality.
FlexiSpy's website even provides endorsements from the recently-divorced, who discovered their partner's infidelity thanks to the software.
"Thanks to FlexiSPY I finally figured out my wife was cheating on me with my brother. I had a bad feeling about this for over a year. After the divorce, my life is so much better now. This Could Be You!"
Makes you want to rush out and buy a copy doesn't it?
Installing either FlexiSpy or Mobile Spy requires physical access to the phone, so the authors claim they can't be classed as Trojan applications. But anti-virus companies disagree.
According to the F-Secure blog: "This application installs itself without any kind of indication as to what it is. And when it is installed on the phone it completely hides itself from the user...So yes, FlexiSpy is indeed a Trojan and we have added the detection to our F-Secure Mobile Anti-Virus so that any user who has a phone that has been infected with this Trojan will get a warning that someone is spying on them."
Ollie Whitehouse, security architect at Symantec, was equally damning of Mobile Spy: "Once Symantec has obtained a sample of this application we would likely add detection in our Windows Mobile anti-virus product. Symantec already detects other applications by Retina-X Studios due to what some would describe as their nefarious nature."
For those interested in hearing what was said, as well as when and to whom it was said, various applications exist for recording calls into MP3 files for later retrieval or automatic uploading, and at least one application can even record what's going on in the room at the time: relaying it over an incoming call from a pre-configured number.
The incidence of mobile phones being infected with viruses, Trojans or worms remains pitifully low, partly because of the low installed base (compared to desktop computers) and partly because the networks are much more secure, so until now the usefulness of mobile phone anti-virus software has been very much open to question.
But, depending on who you are and what you're saying, the chances of someone deliberately installing a spying application of this type are much higher than of catching a chance infection, which might make additional defences worthwhile.