Zero because MS would never do that as its another key bonus for them to be able to do it exclusively on 360. Its as likely as Halo coming to PS3.
You can however use browser type messengers on PS3 like ebuddy but they are not really that great...
Or you can install Linux and use it..
Maybe someone will write a homebrew program so you can use it in the future from the XMB but I wouldn't hold your breath.
Robin