I think they only ported it to get it on PS3 before releasing Bioshock 2 multi-platform. They knew it wouldn't sell as well but its worth it to get the franchise onto the PS3 before the next one.
Clearly they were happy to be exclusive (timed at least) when 360 had big lead, now they see most strong multi-platform games sell roughly equaly on each and want to aim for that instead.
I suspect it'll have legs though, particularly once the price drops. It clearly rankled many that essentially the same game can be bought today for much less as a 360 release. While Bioshock is not a budget game offering it on PS3 for less than full price new game would likely have driven higher sales IMHO.
Although if PS3 fans want to make sure that games like Bioshock are always multiplatform they are going to have to get out and buy the damn thing in reasonable (sorry) numbers otherwise MS might well get a chance to make 2 a timed exclusive as well. And while I don't consider myself a fan I will put my money where my mouth is and get it on PS3... I hate timed exclusives more than anything regarding console industry (which still still annoys me more than PC games due to exclusives, etc. which I don't like either).
Try to be reasonable... its easier than you think...
