Wireless printer issue

I bought a Canon Pixma MP620, which has wireless networking capability. I have the printer networked to two Macs through a Netgear WGR641 (v4) router. I have been able to get the printer to work wirelessly from the router. However, the printer works only when the Macs are hooked up to the router via ethernet. If the Macs are connected only wirelessly to the router, I cannot print to the (wirelessly connected) printer.

Here's another way to show the layout in case the above is not clear:

Scenario 1: [Printer] wireless to [Router] wired to [2 Macs] = works!
Scenario 2: [Printer] wireless to [Router] wireless to [2 Macs] = fails!

Some additional info:

-- I can ping the printer wirelessly from the Macs only when the Macs are wired to the router; if Macs are wireless to the router, I can't ping the printer.
-- Even when the Macs are wireless to the router, I can ping one Mac to the other (and vice versa).
-- When the Macs are wireless to the router, I get access to the internet.
-- I have my firewall turned off on both Macs.
-- I am using WPA encryption.
-- I do not have the router set to accept connections from only specified MAC addresses.

Does anyone have a suggestion for getting this to work so that the Macs can be connected wirelessly to the router and still print to the wireless printer?

Thanks in advance.