I don't recall where I read this (I think it was in one of the closed captioning mailing lists) but I seem to remember that (unlike with closed captions) stylistic aspects of subtitles are controlled by the software (i.e., on the BD/DVD), not via the player... indeed (again, if I recall correctly) this was one reason given that some folks who rely on closed captions/subtitles hated subtitles and preferred closed captions.
I don't have a PS3 or know what specific capability you're referring to in that regard, but my guess is that if there is a way for a PS3 to control some stylistic aspect of subtitles, that that is because the PS3 is actually generating them on the fly (programmatically), and dynamically applying them to the (obviously computer-generated) data stream. Just my guess, though, and wholly dependent on the admittedly sketchy memory of technical details of subtitles on video discs, alluded to above.