Degraded RAID does not work with encryption

With Ubuntu 9.10 and 10.04 server or alt CD versions, with several machines, I have tried repeatedly to set up RAID1 with encrypted data and swap partitions (and unencrypted boot partition). It always LOOKS like it works, cat /proc/.mdstat shows the raid arrays, and everything seems fine when both drives are connected. I have set degraded raid = true, so it should boot with just one drive. But with either of the drives removed the system does not prompt for the decryption passphrase and so will not proceed with booting.

The degraded raid works fine if I do not install encryption.

I have searched the web for information about this problem, and see others have struck the same issue.

The only people who claim to have installed encrypted raid successfully on Ubuntu ALL fail to say whether they actually tested the raid by removing a drive. That's the problem - Ubuntu looks like it has set up the encrypted raid correctly, but it doesn't work in degraded mode.

I want to protect confidential data - hence encryption - and I want to have continuity in case of a disk failure - hence raid mirror - but Ubuntu server currently can't deliver that. Surely for a server edition that's an unacceptable situation.