The scenario:

• You managed to backup your previous phone's data using Wammu (or Gammu) to a .backup file
• You want to import previous contacts into your new, Android 4.0-based smartphone

What worked for me:

1. Extract contacts from the .backup file to a single, multi-contact .vcf vCard 2.1 file using Gammu:

$ gammu convertbackup myinput.backup byoutput.vcf
$ file myoutput.vcf 
myoutput.vcf: vCard visiting card

1. Enable USB mass storage mode on the phone, connect PC and phone using USB
2. Copy the .vcf file into the root folder of the mounted file system
3. Make sure data has been written in full, e.g. run sync on the Linux shell
4. Disable USB mass storage mode
5. Open contacts, pick import from USB storage
6. Give it some time, done.