I have exactly the same issue here.
It's a fresh install off PCLinuxOS with Gnome. The phone is Nokia 5800 XpressMusic.
Just like the first post in this thread, I get "Unable to open device /dev/ttyACM"
I have not yet attempted to fix it, due to the fact that I have a program on my phone that creates a wireless zone around it. And I don't like to mess with config files when I don't have to.
I first discovered the problem on a train, with eight hours of boredom ahead of me. Fortunately i found that app for my phone.