Keyshots, oh no! I misread your earlier post about http://blog.keyshot.com/xmlrpc.php?rsd
That URL should not return "XML-RPC server accepts POST requests only".
If it does, that's a sign of something (in your case lighttpd) cacheing things it shouldn't.
The ?rsd URL should return something like what the following URL does: http://en.blog.wordpress.com/xmlrpc.php?rsd
Another user ran into the same problem and it turned out to be their lighttpd rewrite rules that were causing the problem.
They had been using the following rewrite rules configuration:
#Jetpack is broken with these rules
url.rewrite-once = (
"^/(wp-.+).*/?" => "$0",
"^/images/.*/?" => "$0",
"^/temp/.*/?" => "$0",
"^/(sitemap.xml)" => "$0",
"^/(xmlrpc.php)" => "$0",
"^/keyword/([A-Za-z_0-9\-]+)/?$" => "/index.php?keyword=$1",
"^/.*?(\?.*)?$" => "/index.php$1"
When they switched to:
#Jetpack works with these rules
url.rewrite-if-not-file = (
"^/(wp-.+).*/?" => "$0",
"^/images/.*/?" => "$0",
"^/temp/.*/?" => "$0",
"^/keyword/([A-Za-z_0-9\-]+)/?$" => "/index.php?keyword=$1",
"^/.*?(\?.*)?$" => "/index.php$1"
)
Jetpack worked.
The user was kind enough to write up the problem and their solution at http://snat.co.uk/2011/lighttpd-and-jetpack.html
I'm not familiar with lighttpd's configuration, but it looks like some of those lines are specific to their setup and not generally applicable. I think the important piece of the puzzle was switching from url.rewrite-once to url.rewrite-if-not-file and removing the xmlrpc line.
Is your lighttp configured similarly to the "broken" configuration above (the first one)?
Sorry for the goose chase about the other cacheing systems, we should have caught the ?rsd fact earlier.