No server will be safe now, there will be minecart tracks as far as the eye can see (or the computer can render...)
to place a cart;
after having built / found a track step onto the track and press "V"
W speeds up
S slows down
supposedly pressing A or D (left or right) when approaching a track junction will allow you to change tracks. (at least according to the wiki instructions!)
Watch the Downloads section for the release Download
OR head straight to sourceforge OR the manic digger fan blog where you will find links to get the download right now!!!