[Ruby] deploy with no down time?

[ben wiseley]

We're using cap for deploys (to a pound/apache/mongrel based rails env),
which is great, but it results in downtime and our site, pretty much anytime
of the day, has users online.  I've been doing the daily deploys at 3am -
but even then I'm kicking 30-50 users off line.

Anyone have a better solution?  I realize that sometimes down time is
necessary (like large migrations, things that effect objects in sessions,
etc.) but it seems like changing the text in views, simple tweaks to
controllers, etc. should be able to pushed out with any downtime.  I know
that rails caches stuff - so there'd need to be someway to reset that, but,
other than that, I can go in and live hack files on production and it works,
so it seems like there just needs to be something like "cap update" instead
of the complete copy/replace that cap does on your whole site.

-ben