#AWK — 84 characters

    awk '{printf("The hypotenuse of this right triangle is %.3f\n",sqrt($1*$1+$2*$2))}'

e.g.

    $ echo 3 4 | awk '{printf("The hypotenuse of this right triangle is %.3f\n",sqrt($1*$1+$2*$2))}'
    The hypotenuse of this right triangle is 5.000