[Seaside] Comet timeouts with 403 forbidden

[TimM]

In the final quest to write a small comet training app, I've noticed
that after about 10 mins my web browser shows a 403 forbidden error in a
firebug console when I try to make an Ajax call to update something from
the client.

Is this something to do with seaside sessions timing out?  Is there some
setting I need to use or some strategy I need to keep long running
sessions happy (eg a browser meta refresh every 5 mins?)

Thanks so far with everyone's help, and I will try to feed some of the
info gleaned back to comments on the seaside book.

Tim