Comment 4 for bug 700261

Note: I'm writing all this hoping it will be useful to you to understand the whole problem and find a general solution. For me it's no longer a problem, since with the udev rules I can fix my distro myself and call it a day.

Whitelisting would be the perfect solution, but it would be difficult to implement.
Talking to the GPSD developers I discovered that there are devices that are neither in nor out.
For example, the PL2303 is the worst. It is a cheap IC used by OEM to assemble a whole range of products, including USB to serial ports, USB adapters for older cellphones, and GPSes. Don't know if it's used for modems too, but if it is, you're in trouble as well. The bad is that it has absolutely no identification info, no serial number whatsoever. It appears as a "ID 067b:2303 Prolific Technology, Inc. PL2303 Serial Port" with no way to know what's behind. This raises the edge case of an user with two distinct devices, one which is a modem and another which is not (and doesn't withstand AT commnads), that both identify as a PL2303. In this case there's no way to make whitelisting/blacklisting work.

Aside from this edge case, the bigger issue is if these generic IDs such as PL2303 or FT232 should be in the whitelist by default, or not. If they are, some non-modems devices may "misteriously malfunction", since not all users have the skill to track down the issue and unserstand it's because of modem-manager. But if they aren't in the whitelist by default, some modems may not work out of the box...