If a software raid array is created using a partition that previously contained a file system, setup of the raid array hangs indefinitely. Running the same command that sits there in ps output forever on vt2 results in a prompt saying (paraphrased) "/dev/sda1 appears to contain an ext2fs file system .... Continue creating array? _", waiting for your input. I presume the call made by anaconda is waiting for the same input, only w/o any way to get it, thus the hang. Simply adding "-R" or "--run" to the mdadm --create line skips the prompt and happily creates the array.

This was with anaconda 11.5.0.29, and sda1 was formerly a /boot fs of another install, being converted into a software raid partition for a raid1 /boot across two disks.