Problem connecting to public wi-fi hotspot (captive portal)

I run a holiday accommodation Adrift In Golden Bay with wireless internet for all the guests to use. All PC based computers can connect fine, however at least some Macs, iPod Touchs and I-Phones etc cannot connect.
The wi-fi hotspot uses a captive portal type system that is common at many cafes etc where the initial connection is unsecured but the browser is directed to a login page so that the client has to enter their username and password before continuing to browse.
My router Linksys WRT54GL runs X-wrt and Coova so I have lots of options to change the functions on that but I am unsure if a change is needed on that or on the clients devices.
I know it is a common problem amongst Apple users as guests have mentioned this to me but I have done quite a bit of searching and don't seem to be able to come up with an answer to the problem so I thought I would ask the Apple experts on this forum.
Advice would be greatly appreciated.

Gordonz, a problem we have is caused because the apples go looking for a page at apple.com automatically as an 'is the internet there' thing and if it fails the connection is auto disconnected. We stopped that by installing boingo on the apple devices. No need to configure it, just install it. IPhone Software 3.1.3 doesn't fix it.

More info: iPhone 3.0 Wireless Captive Portal support - MacTalk Forums

Thanks Homz, I will try it out next time I get a problem with a Mac user.

I have downloaded Boingo and a guest installed it on his Mac (and it did say that it was running in the background) but Safari still said it coudn't connect to my captive portal so I think the problem is my wireless router.

I realise now that the problem is not with the captive portal but just a connection problem between the router and the Mac via an unsecured wireless network. I added a couple of websites that should be able to be accessed without logging in (ie within the walled garden) and whilst Windows machines connect fine the Macs don't. I am using Open-Wrt software on my Linksys WRT54GL router,
1. ESSID Broadcast is turned on
2. MAC Filter is disabled
3. Encryption Type is disabled as it is only when the client is logged in to the captive portal that a secure connection is needed.

Is there anything that a Mac needs in order to connect to a wifi that a Windows machine doesn't need?