What is different about the Mac ISO image?
In Ubuntu 10.10, we changed the normal amd64 CD images to dual-boot on either BIOS or UEFI systems (UEFI, "Unified Extensible Firmware Interface", is a different kind of firmware found on many newer systems). This was done using a technique known as a "multi-catalog" CD - it contains two boot images, and the specification says that the firmware is supposed to pick the one it can best use.
Unfortunately, even though Macs use a variant of EFI (an earlier version of what's now called UEFI), they apparently can't cope with multi-catalog CDs, and simply refuse to boot them. This left us in rather a quandary: we needed to support UEFI systems, but we didn't want to drop support for Macs either. I therefore created the amd64+mac CD images, which are exactly the same as the amd64 images except that they only support BIOS booting. Macs are happy to boot these in their BIOS emulation mode.
(In fact, the name amd64+mac is a slight misnomer, because it later turned out that some systems other than Macs suffer from a similar problem - but I felt that a more technically accurate naming such as amd64+nouefi would be more likely to confuse than enlighten.)
While I would love to return to shipping just amd64 images rather than both amd64 and amd64+mac, at the moment there is no prospect of reunifying them unless somebody figures out how to make a multi-catalog CD image that Macs can boot. If you're an expert on this, please do contact me by e-mail.
Appears to be very little between them, mostly the beginnings of EFI booting:
$ diff --context=0 ubuntu-11.04-desktop-amd64*list
*** ubuntu-11.04-desktop-amd64+mac.list 2011-04-27 18:06:42.000000000 +0100
--- ubuntu-11.04-desktop-amd64.list 2011-04-27 18:05:44.000000000 +0100
***************
*** 4 ****
--- 5 ----
+ /usb-creator.exe
***************
*** 134,135 ****
- /isolinux/grub/font.pf2
- /isolinux/grub/grub.cfg
--- 134 ----
***************
*** 144 ****
--- 144 ----
+ /efi/boot/bootx64.efi
I've not personally had much luck with the uefi booting, so I'd probably recommend users stick to the regular amd64 releases until uefi is required. (on most models, the regular disks work fine via bootcamp's bios emulation layer)
Coming from someone who installs Ubuntu on a lot of Macs. There were supposedly some difficulties encountered by some of the people who tried to run Maverick LiveCD's on Mac computers. The EFI loader wasn't friendly with it, so the Natty version with Mac support is meant to interact better with the EFI (Mac version of a BIOS) to solve these problems. Aside from that, there isn't a signifigant difference.
I would advise trying the regular downloads first (if you can spare the bandwidth) because I find them to work great on the Macs that I have used them on. If you encounter problems trying to load up the LiveCD's, then the Mac support version is likely what you are looking for.