its a good idea.. it makes u pretty smarter than ur contact..
but if ur contact just opened ur window to check some thing else like ur email add or ur dp or any thing else.. and suddenly he recieved a msg from u then he will might be annoy ur msg if he is busy that time ..
well still ur suggestion is nice if any one can make a plugin then try it....
but i think as msn msngr 7 works .. its not possible to send auto msg when ur contact opens ur window....
correct me if i m wrong..