You need to set up a DLNA for the PS3 to recognise the vids. I have a linksys NSLU2 with the unslung custom firmware installed. This allows me to run the NAS functionality on 2 ext hdd's plus Twonky media can stream to my PS3s. I know the custom firmware is available for a number of NAS on the market.
Here is the wiki for my unslung. Have a read at some of the other links for other drives that can be 'hacked' [Register or Login to view links]