It's not well documented in any one place (come on Wikipedia!), but there are actually a lot of subtle and blatant differences between the various platform releases of the games; most notably in the first Splinter Cell, according to MobyGames, where level design and bonus content differ between versions. While it would have been neat to have been able to toggle between these versions in Splinter Cell Trilogy, the PC ports are probably closer to the original Xbox sources than any other versions, and feature the highest-resolution assets (which are going to be bumped up to full HD in the Trilogy release).
Additionally, Monney notes in a response to a comment that "the online part of the original games won't be included" in Splinter Cell Trilogy -- just the single-player campaigns. While part of the post URL suggests that the collection will be "coming-to-ps3-in-june" (confirming an earlier report), Monney's officially sticking with, "No firm release date yet, but we'll update you guys once we know!"