I would suggest getting your ad-hoc network working solid 1st. Create it on one computer, then another connects. The computers should be able to see each other, share files, etc --- the whole gambit. Once that is solid, then connect the iPhone and it should provide 'out of local network' routing to all.
Update: I just tried to set this up ... again. It's not working

.
As soon as the second computer connects to the ad-hoc network, it complains about another computer having the same IP and the PDANet status screen changes to the second. I'm using version 1.43. It was a far earlier version (maybe 1.13 or earlier) when I first tested this. I remember reading about a change where the ip was fixed. Let me investigate further.
Update 2: Geeze, I hate firewalls !!! (haha)
Bottom line, I was unable to get PDANet v1.43 to host multiple computer. However, there are many alternatives to make it work. In both examples below, your PDANet is the router for the main computer --- set up normally. To get other computers to use that connection:
1. Enable sharing for the wireless connection.
2(a). Connect 1 other computer through the ethernet port .... or multiple computers through an ethernet router.
2(b). Instead of ethernet, a standard wireless router and setup could be used for the other computers, and the router connected (via ethernet) to the main computer.
3. Disable your firewalls until you get everything working. They are necessary, but are a big pain-in-the neck.
Just finished testing this ... it does work. It could be used for almost anything and almost any number of computers, either wireless or hard-wired. I still believe my initial example will work ... and PDANet can't stop it from working ... however, I don't have the time (or patience) to get to the bottom of this. Think I haven't set up routing tables correctly.
Remember, the primary computer (with wireless and ethernet) must be powered on and connected via wireless ad-hoc to the iphone for anything to work. Also, enabling connection sharing should work without mods, but if it does not, try enabling IP routing in XP systems: “HKEY_LOCAL_MACHINE\System\CurrentControlSet\Servi ces\Tcpip\Parameters”, find a value called “IPEnableRouter”. Double click it and set the value to “1”. For NT systems, it's available in the properties page. For Vista (???) -- sorry I have no idea.