#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