I currently have no Solaris box to test this against and am currently short on time but I'm of the opinion that because of:

   > Sendfile on Solaris can raise EINVAL if offset is equal or bigger than the size
   > of the file (Python expects that it will return 0 bytes sent in that case).

...and the fact that beta1 will happen soon it's safer to use sendfile() on Linux only. Googling for "solaris + sendfile" also raises suspicions that sendfile() may be broken on Solaris. shutil.copyfile() is too central to risk breaking it. 

We may also want to look at socket.sendfile() implementation and add tests for large files for both functions (socket.sendfile() and shutil.copyfile()). I'm adding that to my TODO list.