It's less of an assumption than an inevitability methinks. While true that not all high profile games get transferred, the fact that this is a 1st party game from a massive franchise gives it that much more chance of being brought over. It probably won't happen for a good long while, because they obviously want to sell it on PSP first.
Based on prior trends--Syphon Filter DM, Ratchet & Clank SM, Twisted Metal HO (all Sony games)--we can assume that it'll take a year or more to come out. People who really want to play the game will get it for PSP. By time the PS2 version rolls around, the people who had it on PSP prior probably won't care about the exclusivity since it's a year-old game. Unless they add more stuff to it like they did with Twisted Metal, then they'll probably buy it again.
And I'm sure the GoW3 team (who I'd imagine has little to do with this game) would enjoy the extra monetary boost from a PS2 version down the line.
(SPOILER)Games like Silent Hill and GTA, while also having PS2 ports, are not Sony games. Also, some games like Yu Gi Oh forego the whole facade and just release it on both at the same time (though Yu Gi Oh does something special with it)
sage: Of course, everyone can be wrong.