As my comp.sci instructor would say, "Bad Times".
pptpd: GRE: Bad checksum from pppd.
pptpd: CTRL: Received PPTP Control Message (type: 15)
pptpd: CTRL: Got a SET LINK INFO packet with standard ACCMs
pptpd: GRE: read(fd=7,buffer=80505a0,len=8260)
from network failed: status = -1 error = Protocol not available
pptpd: CTRL: GRE read or PTY write failed (gre,pty)=(7,6)
The vpn connection worked from a direct-internet-connected host, so logically the VPN setup was probably right. Here in the office I am behind a Linux IpTables firewall, and I deduced that it was probably the issue. Nothing was set to block the GRE protocol though, so I was a little puzzled.
I found this TLDP post (Do not bother reading it...) on how to make pptp work through iptables. Ah-ha! It was the firewall. Then I realized that documentation was ancient. By Ancient I mean it refers to the 2.0 and "new" 2.2 kernel. The current Linux kernel is > 2.6! eep! It was probably translated from some obscure dead language it is so old.
Anyway, the solution is much simpler... Iptables has a module that allows pptp to pass through NAT. My tale of woe happily resolved with this command.
One more useful tidbit. You can see a list of available IpTables modules with this command..
locate netfilter | grep .ko