Can I setup a single Sparsebundle in RAID 1? My goal is the following: use my iMac as a TimeMachine Server (under OS X Lion Server) to backup my MBP via the network.

What I did is to choose, in the TimeMachine Server configuration, the internal drive (1TB) of the iMac as a target for the backup.

However I would like to have redundancy, not for the whole iMac data, but for that backup.

If I backup the sparsebundle created on my iMac (using CCC for example), then I'll have to copy the entire content all the time.

So would it be possible to create a virtual RAID 1 drive whose two drives are two sparsebundles, one located on the iMac internal drive and the other one on an external drive.

I think that would be a very interesting solution for what I want to do, and it would be possible to scale it for 3 drives.
You can setup a RAID 1 mirror over FireWire and connect the internal/external drives. However, I wouldn't recommend it.

First, you'll need the same size drives to do the mirror. Second, the speed of both drives will be limited by the type of connection to the external drive, so if its USB, the computer will run at USB speeds. If you are just using this iMac for backup, that may not be an issue, but it's still going to slow everything down.

RAID mirroring is intended mostly for much faster eSATA connections, as that keeps everything at nearly native speeds. What I might recommend in your case is to buy an external RAID and use that for backup. That way you aren't locked in to the internal drive on the iMac, and you can always move it to a different computer if need be (especially in the event of a restore, as it'd be faster plugged directly in than over the network.