All still working good with the anniversary release.
After reading all the post I can find about Eye-fi and ML I think that the problem most users are encountering is that the firmware update can't load from Eye-fi card.
Since now most of them use/know only that method to make the card bootable they can't boot ML even if it's working with other SD card.
The problem is that even if boot flag is enabled in camera, the card is not bootable so ML don't load at startup time.
To solve this problem, as already reported, one can use the command line but there are also programs for mac or Win (maybe also Linux) that can do that without using the console.
For windows there is
EOSCard that can make your card bootable, for Mac there is
MacBoot, and for Linux or Mac there is a script
here.
So far what I have found from other users reports is that on 600D are working:
Mobile X2 (8GB) can't load firmware update (make bootable from computer not tested)
Sandisk Eye-fi 8GB not tested.
Sandisk Eye-fi 4GB (Class 4, for europe only I think) working on my 600D but can't load firmware update. Card format without deleting ML is working and also draw transfer with the cr2toavi trick in ML.
I believe that all the X2 card can work, it's just a matter of boot flag that avoid ML to load.
So please if you have a eye-fi card try to make it bootable with one of the above programs and report back your results.