Maybe this is one of the reasons we keep seeing delays in PS3 games - publishers want to wait a little bit until there are more PS3 on the market, like after the PAL launch.
I have no ideea how easy or how difficult porting was/is - but there were many kind of "ports", so of them were good ones, some were very bad.
The point is, developers cannot exclude PS3 from their plans. They cannot say, at this point, barely three moths after PS3 launched in limited teritory : "PS3 is done this time around". Nobody knows anything at this point, PS3 might fail, but it might also get PS2 like sales. If the later proves true (and its difficult to dismiss that after two generations of over 100 mil on the market) they want to be there to enjoy a piece of the pie. So a dev/publiser has to invest in the platform now (if it hasnt already).
Since games dont tend to get made in a month or two these days, it's clear that everybody has a game or two for the PS3 in the making at this point, that will be released at some point in the future - some of these games will get out on a smaller installed base, but you have to take into account also that competition (in terms of games available on a platform in its launch/first year stage) is also much less acute, so you might stand a good chance to sell that game since there are so few to choose from.
Investing in a platform goes beyond the first game, from a publisher point of view. Everybody know that there arent many consoles out yet, but they will be, and your next games for that platform will benefit from your first experience. Thats what happened with xbox1 - there were very few of them in the beginning, and even after one year, things were not exceptional saleswise - but publishers and developers kept investing in xbox and managed to make good money out of their games later on.