What is the difference between Multi-DB and HyperDB from Automattic?

I've been looking at your Multi-DB plugin recently and have been considering using it, but then I found out that Wordpress 3.0 has this functionality built in. According to Automattic, you simply need a config file from http://wordpress.org/extend/plugins/hyperdb/installation/ and after telling wp-config to enable db configurations it supposedly it does something very similar. Can anyone tell me what the difference is between these two setups? Also, if WordPress supports this functionality already, does this tool extend it? Sorry if I just missed something obvious here...

Thanks!
JohnS