here is a link to the RHEL manuals - which are close but not identical to Fedora.
The short story is this. The mdadm RAID driver system is excellent for making a single RAID partition from several physical partitions. So for example you can take 2 - 100GB partitions (obviously preferable on seperate spindles), say /dev/sda1 and /dev/sdb1 and make a RAID1 mirror from these with a simple "mdadm --create --level=1 ...." command, aand this produces a single logical partition /dev/md0.
By default you cannot partition this 100GB mirror into separate partitions. You can get around this default behavior in two different ways.
You can use the mdadm command with a "--auto-part4" option for example to create four partitions within md0 *but* this sadly requires some information from a userspace config file "/etc/mdadm.conf". This means that unless you enjoy hacking the mkinitrd scripting you cannot boot this partitioned mdadm mirror. The Fedora boot/install does not support partitioned mdadm. Of course you can create a separate 20GB root"/" mirror from a smaller (sda1+sdb1) and another /dev/md1 mirror from (sda2+sdb2) and so on. The Fedora boot and installer can handle UNpartitoned mdadm mirrors.
Now once you create a mirored /dev/md0 block device you CAN create an LVM (logical volume manager) on top of the unpartitioned /dev/md0. LVM allows you to slice and dice block devices in a most impressive way. It's very useful for data server type apps. You can add additional block devices when you need more space, snapshot & remove and migrate the logical "partitions". The Fedora installer and boot-up support LVM on top of an UNpartitioned mdadm RAID1.
Also there is a third option to consider. The LVM manager has both a striping (RAID0-like) and a mirroring (RAID1-like) options when constructing logical volume groups. In my (very limited) tests the LVM striping is a bit less efficient that the mdadm RAID0 - but only a few percent. In addition the LVM scheme has a huge advantage in terms of flexibility and extensibility of storage management. The installer/boot-up will handle mirrored LVM, but you WILL need to create these in advance (before Fedora install) from a FedoraLive CD. The installer sadly does not give you an LVM/stripe or mirror option but instead uses neither by default.
Note that there is a very nice gui based "system-config-lvm tool for handling LVM. It's installable as a separate package and does appear in the F8 live DVD>
My personal opinion. If you wanted RAID5 or RAID10 then the best approach is to create (the no-partition) using mdadm and build an LVM on top of /dev/mdN. IF you want RAID0 then the best approach (IMHO) is to accept a modest (~2%) performance loss and use LVM striping instead.
RAID1 is *the* problem case. Using LVM-mirror (and no mdadm) is perfectly acceptable in terms of performance and very flexible in terms of storage management. The big problem (and I know this second hand only) is that it's a headache reconstructing a broken LVM-mirror vs reconstructing a broken mdadm RAID1. The whole point of RAID1 is data redundancy and therefore security, so I'd personally opt for implementing a RAID1 as mdadm RAID1 (no-partition) and placing an LVM structure on top. Modest performance hit, decent flexibility, good reconstruction profile.
Anyway that's my opinion,