My general line of thinking is that a third party exclusive on either the ps3 or 360 will probably go exclusive unless 1. it is very niche (Valkyria Chronicles, Culdecept Saga - but even then that is not always the case, ala the Way of the Samurai 3), 2. if it sells very well in predominantly one region (Yakuza), or 3. it was just a cheap port from PC to 360 to begin with (the Battle for Middle-earth II).
However, if it's an even remotely high profile title, and it's popular in at least two of the three territories, it'll probably go multiplat, like Bioshock. I figured MGS4 would end up going multiplat (and it may yet - they have yet to announce the inevitable re-release of the game), but the MGS series ultimately did go multiplat, thus in a way proving my point.
I would not be surprised in the slightest if Splinter Cell went multplat. Hell, there's already a Splinter Cell on ps3.