YDL + usb keyboard + mouse will make your PS3 work just like a linux box, but a heck lot faster. If you are not running web server, db server, or java development you will be very happy. So far we've tried Apache Tomcat and Jboss seems fine, coding in C and Java are great. However, since the available memory for jvm is usually only around 200mb, some J2EE package will crash the box. There is also no working 64 bit jvm available that I know of, so you will be using the 32 bit one, it's nothing wrong I think but just not as fast. Sun does not seems like they would build a 64 bit jvm for the cell processor at this time, they want you to buy their servers.... I've never put hd content other than bd and games with ps3, so maybe someone can answer that? swap out the 60gb with one as big as you can get, won't hurt.
I don't think OS X would work right out of the box on PS3, same reason Windows wouldn't, there are a few more free linux builds out there that have been proven to work on ps3. I suppose Apple would run a new build for their OS for cell processors someday??