http://www.google.com/support/a/bin/answer.py?hl=en-ch&answer=60764
You can get the up-to-date range of IP addresses used by Google to send mail by querying Google's DNS for its SPF records:
$ dig txt _spf.google.com | grep spf
_spf.google.com. 300 IN TXT "v=spf1 ip4:216.239.32.0/19 ip4:64.233.160.0/19
ip4:66.249.80.0/20 ip4:72.14.192.0/18 ip4:209.85.128.0/17 ip4:66.102.0.0/20 ?all"
