Set-Cookie: key=value
Set-Cookie: key__Xsignature=signature_string

It seems like this could address a couple issues at once.

* There's a clear distinction between signed and unsigned cookies, so
request.COOKIES can be populated with only valid cookies

* The key/value pair remains unchanged, so things like Google
Analytics can happily ignore the signature if it was applied
unnecessarily (middleware is back on the table!)

Since there may be an upper limit on the number of allowed cookies,
though, doubling the number of cookies could present some very real
problems. RFC 2109 recommends allowing at least 20 cookies per domain
name, and it looks like at least Microsoft takes that to be a
maximum,[1] so it could present very real problems (middleware is back
off the table!).

I'm not sure how many cookies people use on a regular basis, and this
would only be for explicitly signed cookies, so maybe it'd be okay,
but it's flirting dangerously close to being completely unworkable.
Worse yet, it doesn't look like there's any predictable way to know
which cookies would get lost in the event of having too many, so this
may end up causing some very weird application errors if things go
wrong.

At least now it's been recorded for future reference. (Hello, future
me, looking up information on why we did things the way we did! Do we
have flying cars yet?)

-Gul

[1] http://support.microsoft.com/kb/306070