Accessing media files from a mac makes more sense if you have an iMac or a mac mini, since these are always in the same place, may be on all the time and be always connected to an external drive (if you need to).
Accessing a portable mac (in my case, my MBP) seems to me not very convenient. First my MBP only has about 80 GB of HDD space (the other 40 GB is shared with windows), 45GB of those are used by iphoto s library , 15Gb by itunes and the rest by programs and other files, so storing large video files is out of the question. Then your MBP has to be there (or within range), and turned on and connected to an external drive (if your are using one). I usually keep my MBP lid closed, I assume that the ATV wont be able to access my files if the mac is sleeping. I might still get one (an

TV) but I feel that I will really use its potential when I get a bigger drive in a new imac.
This is why I wish the ATV had a hard drive or at least be able to connect to one.