Comment 2 for bug 700261

Thanks for the answer. Currently I've simply removed the modem-manager binary from my system. Next time I'll upgrade my distro I'll use you advice and locally blacklist the serial adapters with udev.

However, my point is that the idea of opening automatically each serial port and sending AT commands as-if it were a modem is a brittle solution for plug and play. There are a lot of other devices that apper as serial ports, such as GPS, Arduinos, even old printers and plotters, etc. that might not like AT commands.
One such example is this: http://fedetft.wordpress.com/2011/01/07/linux-today-too-much-plug-and-play/#comment-202 where modem-manager seems to interfere with a set-top-box.

Those at GPSD that used to make the same thing after my bug report found a good solution: they open the port for a couple of seconds and listen only, which is way less invasive than sending data. I don't know if this is a possible solution for modem-manager. Another option, if modem-manager runs only after the user logs in (which I don't know) would be a popup the first time a serial adapter is connected.