You can still use the 64x64 tiles on a retina enabled iPhone or iTouch (4th gen), but if you plan on supporting the older devices (3gs and below) you will need to use resources at half their current size. So your iPad and iPhone4 resources should be the same, and labeled with the -hd suffix (i.e. temp-hd.png) while your original iDevices will need files without that (i.e. temp.png). If retina mode is enabled, Cocos will look for files with -hd in the name before using other files without it.
The documentation for Retina resources is here: http://www.cocos2d-iphone.org/wiki/doku.php/prog_guide:how_to_develop_retinadisplay_games_in_cocos2d
The only thing you have to look out for is mentioned in the other thread I linked to, and it's that you may need to use different positions for objects.
I don't know what program you're doing your tile sheet with, but I'm using texturePacker and it has an option to automatically create "SD" (standard definition images) right in the program. I'm not sure if the other programs do or not, but essentially, it multiplies all image sizes by .5
Recap:
You will need to scale your images down only if you plan on supporting devices before the iPhone 4/iPad (non-retina devices, supporting these devices is HIGHLY recommended IMHO).
If you support older devices, all of your resources will need a standard and high def version. In your case, you will need to scale everything down by 50% for older devices, while renaming all of your current resources with -hd
Hope this is helpful and answered your questions.
