SimpleTwitter displays the most recent twitter post (tweet) for a given user. The tweet is cached on the webserver for a number of minutes.
Your twitter user id is the user name that you used to sign up to twitter.
Yes. Anything that resolves to http://twitter.com/twitter_id/ can be used.
This will be because of the cache. This will always be a compromise between instantly updating your website with the most recent message, and always serving a complete webpage to your visitors.
SimpleTwitter runs before a page has finished loading. If the cache has expired, a request to twitter.com will be made. Depending on how busy twitter.com is, it may take a while to respond. There is a fairly small time out set in the plugin, which should restrict the time it waits for a response to an acceptable level. If twitter.com hasn't responed in this time, the cache will not update and the old message will be displayed. Remember, it should only have to do this after the number of minutes configured in wp-admin.
A connection has to be made to twitter.com within 4 seconds. A response has to be received within 8 seconds.
Not yet, no.
You can't just yet. This isn't called SimpleTwitter for nothing!