MDADM Versions

distro kernel version mdadm version
Ubuntu 6.06 LTS 2.6.15 1.12.0
Ubuntu 7.04 2.6.20 2.5.6
Ubuntu 8.04 LTS 2.6.24 2.6.3
Ubuntu 8.10 2.6.27 2.6.7
Ubuntu 9.04 2.6.28 2.6.7.1
Ubuntu 10.04 LTS 2.6.32 2.6.7.1
Ubuntu 10.10 2.6.35 2.6.7.1
Ubuntu 11.04 2.6.38 3.1.4
Ubuntu 12.04 3.2.0 3.2.3
CentOS 4.5 2.6.9 1.12.0
CentOS 5.0 2.6.18 2.5.4
CentOS 6.0 2.6.32 3.1.3
Debian 4.0 2.6.18 2.5.6
Debian 5.0 2.6.26 2.6.7.2
Debian 6.0 2.6.32 3.1.4
Fedora 7 2.6.21 2.6.1
Fedora 15 2.6.38 3.1.5

MDADM 2.x on kernels >2.6.17 supports online resizing of RAID 5 arrays :)