via http://groups.google.com/group/twitter-development-talk/browse_thread/thread/6f24e47cca5b68f6

We've successfully moved to the OpenFire Jabber server, and now have
far more visibility into our XMPP services overall.

だから20req/hr制限なのかしら。
それにしても、早くtrackを復活させて欲しいです。
http://twitter.com/account/rate_limit_status.xml

###OpenFireとは
Javaで書かれているそうです。twitterが以前使っていたejabberdはErlang。そしてあんまり情報がない...

Openfire (formerly Wildfire) is a real time collaboration (RTC) server dual-licensed under the Open Source GPL and commercially. It uses the only widely adopted open protocol for instant messaging, XMPP (also called Jabber). Openfire is incredibly easy to setup and administer, but offers rock-solid security and performance.
http://www.igniterealtime.org/projects/openfire/index.jsp

###twitterが以前使っていたJabberServer：ejabberd
http://www.jabber.org/servers/ejabberd
http://www.atmarkit.co.jp/news/200704/27/erlang.html

###Refs
http://www.jabber.org/servers
http://www.jabber.org/servers/openfire