Update: it's seems that the commerce aspect has been removed from the website in order to promote more downloads and feedback on people's plays. It's now totally free to download plays, however I'm still not sure if I'd want to take part. Could there be issues of plagiarism or copyright infringement or are the dangers the same as anywhere else?
The Bush project is legitimate. Alomg with posting your work on their secure site, the Theatre also gives you the option to submit to The Bush for production consideration, as with most theatres it takes a while for them to get back to you, but they do. My experience with them was great and I would recommend them to anyone. http://www.spivsplays.com