Yet it works.
One of the many advantages of software RAID is that you can mix types between partitions on the same drive. When I next reinstall my netbook (for a clean shift to Ubuntu 9.10) I plan to try RAID0 its root partition (over its very slow internal SSD, an SD card that seems to run at much the same speeds for writing (a little slower for reading), and maybe a little USB reader like these with a quick uSD card in), but have /home and other areas where I might want to keep files safer in a RAID1 array built on other partitions on the same drives.