The should be able to talk to one another, but it would be under "ad hoc" mode. This is specifically for peer-to-peer wireless networking, similar to what you would find in a peer to peer or bus topology LAN. While you might need a router, it doesn't have to be a separate device. You should be able to share your connection using Internet Sharing in Mac OS X (I think the earliest version to support this is Jaguar). This basically turns your computer that's sharing the connection into a router. Consider that a hardware router is really a computer that runs software and is only dedicated to that. Any computer that has this support for sharing an Internet connection basically is capable of routing. Windows 2000 and up have this function, as does OS X. (Please don't mention 98 or ME, since the implementation on those OSes are not good.)
My guess is that you would set up both machines in "ad hoc" mode, or "Computer to Computer" mode (as I think it's described in OS X). This way both computers talk to one another (I'm sure I'm missing some other settings, but this is basically the setting you need for the wifi.) Now, on the machine that is going to share the connection, you would need a hardware NIC/modem (depending on the connection type you have)to connect you out to the Internet. Since I'm sure the Mac and the PC laptop would have a built in NIC or modem, that part is already set. If you have DSL and are using PPP over Ethernet and have a DSL modem, you can set that modem to bridged mode and then the NIC on the sharing computer to use PPP over Ethernet and share the connection there. Your gateway address would then be the IP for the wireless on the sharing computer. If you are on a modem, you would activate the sharing on the modem and your gateway will still be the IP for the wireless connection on the sharing computer.
I hope that made some sense. If not, I can provide a graphic to clarify what I am talking about. Even though you don't have a hardware wireless router per se, you are dividing up the jobs that a wireless router would do unto different tasks (WLAN connection would be P2P, while the routing is being taken care of by the computer sharing the connection).