Subvolume feature, using the current Ubuntu kernel (from ubuntu 12.10), doesn't make much sense as it still misses "Subvolume Quota Groups". You have to wait kernel 3.6 (maybe ubuntu 13.04 ?)
You can snaphost a subvolume of course, and of course you can't snapshot a directory. But you probably expect subvolumes to work like Logical Volumes with LVM, but with btrfs we don't have an alike scenario, as we don't have quota extension with this kernel.
Therefore: yes, you can make snapshots, but they look like directories, and if you fill the space on a subvolume, you fill the space on the whole filesystem.....
You can even resize a subvolume, by giving the path of the subvolume, but the resize operation affects the whole filesystem, and not the path of the subvolume.
In other words, with this kernel, subvolume is not production ready.
Otherwise, you have to use btrfs filesystems as volumes, and not subvolumes. btrfs filesystem can be shrinked on-line, and extended on-line, and therefore quota is not necessary.