This really annoyed me and I fixed it, so I thought I'd share.
I use gvim under Ubuntu to do all my coding. gvim associates *.ss with some other file type, so the syntax highlighting wasn't correct and it wasn't auto-indenting at all.
I edited /usr/share/vim/vim71/filetypes.vim - do a text search for "Scheme" and you should see this:
au BufNewFile,BufRead *.scm,*.ss setf scheme
For me it was on line 1670.
Comment it out with a double-quotation (") and load a .ss file - it'll now use the xhtml file type for syntax and indentation instead.
There's probably a better way to do this... but I've got work to do!
I hope this helps someone :-)