WordPress.org

The Twittermail function does not work

You need an Email service on your Webspace. It won't work with Mailservices like Googlemail oder Gmx.

It takes some time until the message arrives on twitter

Thats a problem with your Webhoster. For example on byethost, it takes up to 30 minutes until your twitter status is updated.

Whats the difference between Ping.fm API Key and Ping.fm User Api Key?

I am still waiting for the plugins approval trough Ping.fm. If they approve it, you only need your User Api Key. Until then, you must request a developer Key.
You can get your User Api key here: http://ping.fm/key/. The additional API Key is required to enable the communication between Schmie-twitter and Ping.fm. You can make the request here: http://ping.fm/developers/. I made some Screenshots of this procedure. Have a look at my page: http://schmiddi.co.cc/wordpress_plugins/.