Table returning functions aren't supported at the moment, the main reason is that there's no facility to specify the layout of the table returned in a way that our query system will accept it.
The main reason for that is: what to do with a table being returned: is that the resultset or is that a variable (== single value)... the resultset route could be solved with additional code perhaps, the other one, not really...
Additionally, there's the problem with specifying mapping info for each column.
I also don't expect us writing support for this in the near future.