I've finally found the holy grail:
http://www.jcsilva.com/2011/01/09/issues-with-dd-wrt-gre-forwarding-pptp/
This page says the following:
Issues With DD-WRT GRE Forwarding PPTP
A quick fix for DD-WRT not forwarding GRE PPTP packets.
Add the following commands to the startup commands (Administration->Commands):
/sbin/insmod xt_connmark
/sbin/insmod xt_mark
/sbin/insmod nf_conntrack_proto_gre
/sbin/insmod nf_conntrack_pptp
/sbin/insmod nf_nat_proto_gre
/sbin/insmod nf_nat_pptp
As a side note, I'm not sure why the Professional machine was able to connect correctly and at this point I don't really care now that I have a real solution that isn't enabling the DMZ.
The problem is not with your routing table, it looks just fine. What is not correct is however your configuration file, and there might be a problem with the configuration for use with IPhones. I suggest you make a copy of the current file, to be used in case of disaster, but then let us proceed to write a new one.
You can check the proper way to configure your VPN here, but I will walk you through this.
I think the first error is in the WINS2 address. You used an OpenDNS address, while WINS2 should be an address within your LAN; from this I deduce you do not have a WINS2 server on your LAN (most people do not, I certainly don't), so just drop this line. The DD-WRT manual link above states:
WINS Add your local WINS servers. This settings are optional.
(Emphasis mine).
Then the server IP is wrong; the guide states
Server IP. Your LAN IP Address.
You can use your router's LAN address, 192.168.1.1
Then there is the client IP address range. The guide states:
Client IP(s)
The client IP range. Leave it blank will not work. ... Should be a valid IP Address on the LAN segment of the network.
Thus it should be within the range 192.168.1.0/24. Please pick a range, not just an individual address, because syntax may be sensitive to the lack of a dash.
Lastly, the peculiarity of IPhones. The same guide, a tad later, here, states that there is a DNS problem with IPhones, and that addition of a few lines to the router start-up script will cure this problem. This point is a bit dubious, because it is not clear whether this DNS problem affects all IPhones, or only some versions of IOS (and thus no point in my asking which one you are using). Thus I suggest you try your VPN without this correction first; then if it fails, you set it up as per the link provided.
Best Answer
Forwarding port 1724 isn't enough for PPTP, you also need a router that can, and has been setup to, forward GRE (protocol 47) inward to the VPN server.
Note: This is NOT "PPTP Pass-through", as that's for allowing multiple people behind the router to access outside PPTP VPNs.
From what I can see in the e1200 simulator, if you're using firmware 2.04 (or later) "PPTP" becomes and option in the Applications & Gaming -> Single Port Forwarding dropdown for Application Name.
I would suspect that using that predefined "PPTP" entry will also enable handling GRE, but it's not obvious, and I don't have one here to actually test with. :)
Beyond that, you may want to look into loading a compatible 3rd party firmware onto it (like, say, DD-WRT), which can/will give you more options for configuration.