Anyone have a definitive list or info on network operators who definately
allow or definately disallow GRE packets across their networks.
Sorry for the semi-operational content 
It's hard to imagine any serious network providers who would
block arbitrary kinds of traffic.
Anything you'd hope to gain by such an action seems futile anyway --
the people you're trying to stop could always tunnel through some
other mechanism.
--jhawk
  (we certainly allow GRE packets and expect everyone else does, too)
Several others have posted replies on this topic, but they've missed the most
common situation. I've seen (major) network providers with the following
access rules in their routers:
  allow tcp
  allow udp
  allow icmp
  deny *
While not explicitly blocking GRE, they're implicitly dropping everything
(including IPsec traffic, which is how I found this; my corporate VPNs weren't
working :-).
Of course, trying to get this resolved took *weeks*, because I couldn't talk
to anybody who understood that there were protocols besides the ones listed
above... *sigh.