Disclaimer: I'm not a lawyer, and you shouldn't treat this as
authoritative legal advice.

> First of all, the "evil" thing about GPL is that anything that uses
> _any_ GPL stuff (except libraries or whatever) must be _entirely_
> placed under the GPL, right?  (From clause 2b of the GPL.) And thus
> the reason we (that's the collective NetBSD-we) don't want any GPL'd
> code in the kernel source (because it would then "contaminate" all
> the other code), right?  Or am I missing the point?

It wouldn't exactly "contaminate all the other code," since it
wouldn't prevent you from distributing all the other code under some
other copyright, but it would prevent anyone from, say, distributing
kernel binaries under terms more restrictive than the GPL, or
distributing kernel binaries without providing complete source.  The
NetBSD project would like to let people use code from the project for
essentially any reasonable purpose, including as part of a commercial
product.

> Second, is there anything wrong with using BSD-copyrighted stuff in
> a GPL program?

My best interpretation of the copyrights is that you cannot distribute
a program (or kernel) using both BSD-style copyrighted code and GPL'd
code, because the BSD-style copyright adds restrictions (however
minor) to the GPL.  Naturally, if you wrote the GPL'd code, you could
distribute it under a modified version of the GPL which allows those
particular restrictions.

Not everyone agrees with this interpretation (in particular, I don't
think people from the FSF generally agree with this interpretation),
and certainly not everyone holds themselves to it; for instance, the
Linux kernel currently ships with some code with BSD copyrights (the
SLIP and PPP code, for example), which is possibly in violation of the
GPL on the rest of the code.

> Third, is there any reason why I would not want to place my own
> original code under a pick-and-choose copyright (BSD _or_ GPL, at
> the user's discretion)?  That's what I was planning on doing, to
> make other people's lives just a bit easier...

Perl does something like this; you can distribute Perl under either
the GPL or the Artistic License, which I think is a little less
restrictive than the GPL (I've only skimmed it, though).  I know of no
reason not to do what you suggest, unless you want to prevent your
code from being used in a proprietary product (which is the primary
goal of the GPL).