Another thing that's not very clear from the document is: if the signal arrives in the course of a non-interruptible system call, when will it be delivered? As soon as the system call ends or anytime in the (near) future? Probably POSIX does not define this behavior, but it's useful for the programmer to know how most operating systems handle this.

Another interesting thing that I had to read in the Linux source code to be sure is that non-fatal signals can't interrupt a write(2) against the filesystem (and you can't have a short write count returned because of a signal). This is an assumption many programs rely on.

The Linux man page on signals is surprisingly helpful in that regard: http://linux.die.net/man/7/signal