I think RCI changed things with their wi-fi over the past year. I am a Mac man too and as recently as 2007 I had no problem using my Mac on the Liberty Of The Seas. Last year on the Rhapsody it didn't work, nor did it work on the Sovereign later last summer. I ended up taking a Dell laptop I have on the Grandeur in June and it worked fine. I took a Dell netbook on the Celebrity Equinox a couple weeks ago and it worked, although generally the ship had wi-fi problems because it was free and the ship was full of journalists. However I was pleased to see not only people using wi-fi with their Macs, but the internet cafe also had four MacBook Pros set up at some of the stations (all the internet cafe computers were laptops).
It could have been just some issues with the Rhapsody and Sovereign having internet problems. Really there is no reason a Mac shouldn't work with any wi-fi signal, although I have had problems with it not working with all Mac browsers. One time I had to log in with Firefox and then I pulled up Camino to access the internet. I happen to love Camino. Great browser.
As for roaming...I normally don't use a cell phone when traveling but in the UK case I was alone and my family back home, so they wanted me to check in periodically.