I don't have 12.04.1 LiveCD... But following is from 12.04_amd64 LiveCD.
(1) Without ARCH partition mounted: ubuntu@ubuntu:~$ sudo os-prober /dev/sda1:Ubuntu quantal (development branch) (12.10):Ubuntu:linux
(2) With ARCH partition mounted: ubuntu@ubuntu:~$ sudo os-prober /dev/sda1:Ubuntu quantal (development branch) (12.10):Ubuntu:linux /dev/sda3::Arch:linux
The problem also persists with 12.10_amd64 Beta1
I don't have 12.04.1 LiveCD... But following is from 12.04_amd64 LiveCD.
(1) Without ARCH partition mounted: :Ubuntu: linux
ubuntu@ubuntu:~$ sudo os-prober
/dev/sda1:Ubuntu quantal (development branch) (12.10)
(2) With ARCH partition mounted: :Ubuntu: linux :Arch:linux
ubuntu@ubuntu:~$ sudo os-prober
/dev/sda1:Ubuntu quantal (development branch) (12.10)
/dev/sda3:
The problem also persists with 12.10_amd64 Beta1