Simply using while (read-line ifile nil nil) seems even more compact.

  However, I have this negative gut reaction to wanton waste, as in
  effectively allocating as many strings as there are lines for no
  good reason, just exercising the garbage collector, so while we're
  at it, how about

(loop for char = (read-char ifile nil nil)
      while char
      count (char= char #\newline))

  Note that read-line will return nil on an empty file, but the data
  leading up to the end-of-file if no newline intervened and then nil
  on the next call, effectively counting a non-empty file containing
  no newlines as having one line.  This may be relevant.

| (loop for line being each line of ifile
|       count 1)

  Elegant.