I have just released a new version of mdadm - 1.6.0.
Mdadm is my tool for managing Linux Software RAID arrays.
This release included initial support for a --grow mode that allows resizing on-line arrays. Naturally this requires kernel support to be able to work. The 2.6.7-rc2-mm1 Linux kernel has support for changing the active size of component devices and changing the number of drives in a RAID1.
I hope to add support for adding drives to a linear array soon, and adding drives to a RAID5 eventually.
I had hoped to get support for the new-style superblocks into this release, but it just didn't happen. Maybe next time.