How can I format a BTRFS Raid 1 mount?

I found the following on the btrfs wiki, specifically the Gotchas page (quoted verbatim):

• on a -d raid1 volume, df will show total raw space, space used by data (not counting duplication), and the difference between those two numbers. This can be surprising since you'll run out of space before the Use% reaches 50. (up to and including 2.6.33 -- reporting changed in 2.6.34)

• In 2.6.34, df will show total raw space, space used by data (factoring in duplication for raid1), and raw free space, taking metadata and data into consideration. You can still hit an "out of space" condition well before free space reaches 0, especially if, under raid1, your allocated space is unbalanced, and the system finds it impossible to find space for your new write on two separate disks.

Since you're running Ubuntu 10.04, which uses Linux kernel version 2.6.32, it appears you're seeing the "surprising" behaviour described by the first bullet point. So, in all likelihood, the filesystem you created really is RAID1, df is just over-reporting its size.