We usually recommend the software we write about, but thats not always the case. As a parent I could not recommend using your Windows Mobile smartphone as a baby monitor, but nevertheless, the software exists, and is pretty interesting.
Baby Monitor for Windows Mobile, written by Russian resident Pavel Ananyev (where child protection laws must be less strict than here) allows you to use your windows mobile smartphone to detect noise made by a baby waking. It features advances noise suppression software which allows it to ignore environmental sounds which could trigger false alarms, and automatically silences the ringer, so your child wont be woken by an incoming phone calls.
On hearing a sound the phone will dial a pre-selected number, and you can listen in and confirm your baby is indeed awake. The author commends his solution, as it prevents issues with interference related to radio noise which plagues devices which run on the unlicensed spectrum that most baby monitors use, and of course features an unlimited radius of action, meaning you could be anywhere in the world (!) and still hear when your baby wakes up.
The software is $19.99, which may be the last money you spend on your child after the social services comes to take him or her away, but does feature a trial version.
Download Baby Monitor for Windows Mobile here.