you have two options:
forget procmail!
a) follow the maildrop filering guide and implement an autoresponder by piping the mail from maildrop .mailfilter file or
b) take a look at Yaa (
http://freshmeat.net/projects/yaa/).
I have plans to cover these very soon and write a guide for autoresponders so if you can sit tight, i'll try and get something up asap for you.