Indexing on a separate volume

I was thinking this morning, how much work would it be to change the directory
structure of Postgres. 

from:

postgres/base/    db1/    db2/    db3/    ...pg_xlog/

to

postgres/base/    db1/    db2/    db3/    ...index/    db1/    db2/    db3/    ...pg_xlog/

Or
postgres/base/    db1/        data/        index/    db2/        data/        index/    db3/        data/        index/
  ...pg_xlog/
 

This sort of arangment would allow putting indexes on separate volumes. If it
is a stupid idea, with little value or a huge amount of work, I take it back.
Seriously, however, does it make any sense?