I used my PS3 last year almost exclusively to watch MLB.TV on it. There is an application that you load onto your PS3 then you sync your MLB account and it works.
The picture quality is okay, I wouldn’t say HD quality.
I experienced issues a few times with wanting to wait about until the 3rd inning before I started the game – so I could fast forward through commercials until the end. Not sure if they have this fixed, but several times I would get to the end of the game (8th/9th inning) and it would stop loading. I would then need to wait until the posted the archive copy and then start it from there.