[rgnitz]

Indeed you can distribute data to multiple independent RDBMS, but balancing when new nodes are added is probably a manual process (or a lot of custom code) that is likely to require downtime. To avoid downtime, your application would need to write to both chunks while it is balancing/migrating (and then delete the old data/chunks once it is migrated to a the RDBMS). Essentially, you would need to write what is already in MongoDB.

You would also have to write a parallel query engine.

I too am a fan of simple designs, but I think rolling your own sharding on top of a RDBMS would likely be a massive chunk of time.

There are really expensive commercial products working on horizontally scaling RDBMS... but personally, I prefer open source and document oriented databases :-)