IP configuration on the wwan0 interface failed on my Lenovo Carbon X1 with Kubuntu 13.10 (3.11.0-15-generic). The problem seemed to be a due to a kernel change;
I followed the proposal made at the end of the bug report; add
options cdc_ncm prefer_mbim=N
That solved the problem!
And while I was at it, I also added
No reason to have avahi running on an interface towards the Internet.