PS3 Media Server is a java application that plays movies on your PS3, Xbox 360, or other uPNP client from your Mac. It's free. It works. It isn't as purdy as getting an AppleTV, but it does the big part of what you're asking for, and it does it competently. It'll even transcode movie formats that aren't compatible with PS3/XB360 into formats they can use. Keep in mind though, if your PS3 is one with a 'g' card in it, it may be slow for HD video.