(If you don't want to bother with the history lesson, just skip straight to the answer)

Rails has traveled many different roads to deployment over the past five years. I launched Basecamp on mod_ruby back when I just had 1 application and didn't care that I then couldn't run more without them stepping over each other.

Heck, in the early days, you could even run Rails as CGI, if you didn't have a whole lot of load. We used to do that for development mode as the entire stack would reload between each request.