Macs are really fantastic. Their hardware is high-quality and reliable, and their coded apps are great. I highly recommend them to all who ask. But you really have to examine what you want to use your mac for. I think if you're going to buy a mac and run ubuntu on it, it'll be a waste of money, and you'll struggle with compatibilty.
You still didn't answer Spr0cket's response. Why do you want a mac?
I love mac. I love ubuntu. I just think that ubuntu on a mac is paradoxical.
If you're looking for a high end PC to run ubuntu, check out the Dell XPS series - You can buy them with ubuntu pre-installed. If you want a PC which looks even cooler than mac, and you can afford it, check out voodoo pc.