I would be good to support this useful rails plugin. I thing these two things would be sufficient:

1. Add new folder "Selenium Tests" or something to Projects window when rails application project is open (maybe only
for applications that have selenium-on-rails installed).
2. Associate *.rsel files with ruby editor

But i think both things could be done in more general way: 

1. Add possibility to define custom folders for project
2. Add possibility to associate any file extensions with any editor (i though this is possible through Tools > Options >
Miscellaneous > Files, but there is no ruby file type available. This might be a bug actually)