Warehouse Builder is a cost-free(part of the database license) ETL tool which produces PL/SQL when its mappings are deployed. The components you use to develop your data flows are based on Oracle’s SQL and PL/SQL capabilities.
In my opinion you really don’t need to own a warehouse to use OWB, with 11g now it is even coming installed with your database. So if you are also an Oracle database minded developer you better plan to check OWB 11g, similar to Apex I can guarantee that you will be having lots of fun and most probably some opportunity to develop your knowledge :)
Here is a hint showing how it can be possible to automate generation from your standard SQLs to OWB mappings.
And here you will find some important OWB hints.
Extract(Read) > Transform(Test&Apply) > Load(Learn) Blog 