Canonical is thinkig of making the 64 bit operating system as the default one, recommending the users to download the amd64 distro, starting with Ubuntu 13.10 Saucy Salamander. For now, the 32 bit Ubuntu systems are the one recommended to the users.
This may be a good decision, since the 64 bit system is needed by UEFI. Also, the 32 bit applications work on 64 bit systems, while the other way around fails.
The Ubuntu Server 13.04 comes already by default in the 64 bit flavour, unless the user manually selects the 32 bit version. (from the mailing lists)
Chakra and Fedora already recommend the users to download the 64 bit versions, while Arch Linux is intending to switch to this trend.