Hi everybody, i resolved my problem with udpcast and commercial version of ssh, and i'm posting here the solution to save some time to anybody with the same problem. I found that ssh2 (3.2.9.1) needs a couple of options to work the way i intended in the previous post:

for i in `seq 0 9`; do ssh -t -f machine$i "udp-receiver -f /dev/hda1"; done

where -t "allocate a tty even if a command is given" and -f forks the process into background.

OK now i can clone that 73 machines classroom... ;-)

Thanks to everybody and expecially to Alain for writing udpcast (ghost? what is it? ;-)

Ciao, Matteo