If you change the IP in the packets, how can someone 'call you back' with a packet? It would go to the wrong place
yes when u spoof ur ip,the other sys can't call u back with a packet but probably u don't wanna receive any packets.
if u can see the packets which the other sys(whose ip u have used for spoofing) receives then its fine.
like i just wanna see if the sys is up but don't want to use my ip for ping scan then i can use some other sys's ip whose packets i can view.

and Raslin - there's a small utilty tool i came across few days back.i just tried it on my lan.works fine.
called hping2 (some beta version-don't remember from where i downloaded it but guess mr.google will do the job).
see if thats what ur lookin' for.