I've got to be more precise here, obviously.

The license talks about two types of people
(that can be the same of course); contributors
and distributors.  If you want what you wrote
to be a contribution, you must affirmatively
say it somewhere (like email to the Labs
or 9fans containing the contribution or announcing
it to the web).  You are then a contributor and
your contribution and you are protected by the
license should anyone distribute it.

A contributor also is required to provide source
and required to provide a 'royalty free
patent license...' to anyone that accepts the
license.

If you want to distribute something containing
plan9 source or plan9 binaries you are a distributor
(D).  A distributor can distribute under a different
license and doesn't have to disclose any source
that it not a 'contribution'.  However, D's
license must be compatible with ours, i.e.,
must have similar cover your ass clauses
(see the license for specifics, but I believe
most OSI licenses would be satisfactory).

Also, if the distribution is commercial (for
recompense of some sort) the distributor must
indemnify all contributors against suits brought
as a result of actions taken by the distributor
(false claims, bad software, malicious behavior, ...).

If you link your code with our libraries and distribute, we
have taken that not to be a distribution since the
only people that can run it are somehow in the chain
of licensee or distributor so that they would already
have the libraries.  If you don't want to trust us
always being that nice, you can distribute your own
object files without the libraries since the end
user can link anyways.

The salient point is that SOURCE YOU WRITE DOESN'T
HAVE TO BE MADE PUBLIC unless you call it a
contribution.  So if you want to base something on
Plan 9 (including hacking kernel and libraries)
but don't want to give away your work, you can
do so.  However, if you want to make money off of
it, you have to take financial responsibility for
your actions.