How can back up my wifes iPhone 3GS with my iTunes

I'm having problems trying to backup my wifes iPhone......

Basically, she has been using a 3GS that was my sons some years ago.....but now that my Daughter has upgraded, we have an iPhone 4 that she can start using instead.

However, when I connect to my iTunes, although it can see the 3GS, it doesnt give me any option to 'Backup', - which it did recently when I did the same exercise for my Daughters 'upgrade' and change of phone. (4 - 4S)

My iTunes is only giving me the options to 'set up as a new iPhone', or Restore from the Backups of either mine or my daughter's phones.

Is this to do with the software perhaps ?   Or does iTunes limit the number of handsets it will 'sync/backup' with ?

Any ideas/advice gratefully received......

Many thx .

Chris