I go to a lot of various gaming message boards, and I always see fanboys making topics like "[insert PS3 game] will be ported because more people own the 360".  Since when did the number of consoles determine a games exclusivity?  I'm not denying that a large userbase is very attractive to developers and it would be dumb to imply that.  However, it seems like a lot of people think that 3rd party developers publishing exclusives on a system with a lesser userbase is some new phenomenom.  How can people have already forgotten the countless times it happened just last generation?

Tecmo released several XBox exclusives, such as the Ninja Gaiden series and Dead or Alive 3/Ultimate/Volleyball.

Koei released Crimson Sea exclusively for Xbox.

Capcom released Dino Crisis 3 exclusively for Xbox.  They also released the RE0 for the GC exclusively.  Powerstone and subsequent sequels were released exclusively to the Dreamcast.

Squaresoft released Crystal Chronicles exclsuively to the GC.

Viviendi released the Riddick game for Xbox exclusively.

I mean, I could keep going, but I think I've made my point.  Can someone tell me why NOW this has become something out of the norm?

Ever since games started costing a shite load and the developers want to maximize their profit. Its called business. If a game does go exclusive, they probably want huge financial incentives for it. Whats the confusion?

its not that u get exclusivity only if winning, u get a lot more and games that would be exclusive for others arnt because of user base.

dmc is a perfect example. was on ps2 because it had first mover. stayed on ps2 due to user and fan base. its now multiplat because ps3 doesnt have the lead in the war.

It's usually on a case-by-case basis though.  If you'll recall, the dominant console has always had the most exclusives by far--SNES, PS1, PS2, and DS have all shown that.  That said, just because it doesn't have the most exclusives says nothing about the quality of said exclusives.  N64, though it had few games, had more games that people generally recognize as the greatest of all time than the PS1.  PS1 had FF7, while 64 had Mario 64, Zelda, Goldeneye, and Perfect Dark.  When you think about it tha way, while the consoles that have sold less have had less exclusives, it doesn't mean that they are bad ones.

I know you are referring to PS3, so you can rest assured that, though it will likely have less exclusives, in the long run, that says nothing about their potential quality.

Once I saw GTA: Vice City on an XBox cd case, I knew this would be the similar trend to 'a lot' of games, but not all games... every PS2 'fanboy' had preached how PS has GTA series, but that got shot down after a year or so.

It's ironic. Well, not really. That's why first and second party games are what define a console. Everyone else is usually along for the ride. If the game is not distinctive to the platform, then ports work even easier, and ports sell better.

This is different from last generation because of the double-triple development costs. If last gen game required for example 300k sales to break even, this generation (except for the Wii) would require 600-900k sales (my numbers are deliberately high for the example).
Publishers profits are declining, and even more with rising dev costs, and they need to make more money than before (due to inflation etc.), which doesn't fit with rising costs. So, you need to release your game for a bigger install base. There are 3 ways to do so; you can go multiplatform, release the game for the console with biggest installbase or delay the game until installbase for a console is big enough in order to make profit with your game. Now, we are talking about games which takes more than 20M to develope, and the publishers need to get their investments returned in certain time period in order to have money to make new games and stockprice in a certain level. So, the fastest option is to go multiplatform. It doesn't take too long to port a game to a different platform when you have the option of waiting maybe a year or two before the installbase reaches certain level.
This doesn't mean that there wouldn't be exclusives, just that there will be less of them and most will be 1st party or the ones, which development has been funded by the console manufacturer.

If you were hinting towards MGS4 and FF13, i'd guess MGS4 could go multiplatform. FF13 not so much, but it could be delayed a lot.

Monster Hunter 3 went from PS3 to Wii.
Dragon Quest 9 went from PS2 or PS3 (I forgot which) to DS.

This is just the beginning.

 The GTA was ported to XBox as soon as the exclusivity deal with Sony expired. If i recall, new contract considering the exclusivity, was made as soon as the XB versions got their release.

As others have said, when a third party places an exclusive game on a console with a small install base, it's usually due to some kind of behind-the-scenes deal. MrPickles gave some good examples from the last generation; Microsoft paid a lot of money to get third parties to put games on the XBox. (This is one of many reasons why the console was a sales success, but a financial disaster.)

Nor is Nintendo above this sort of action either. The Gamecube only received the Resident Evil series because Nintendo wrote Capcom a big fat check. Final Fantasy: Crystal Chronicles has an even more interesting story behind it. Square wanted to port their old Final Fantasy games over to the hugely successful Gameboy Advance, but Nintendo refused unless Square developed at least one new game for their faltering Gamecube console. Square agreed, and that's how Crystal Chronicles was born. I know the series has its fans, but I'd argue that it's subpar status is due to how it was conceived: Square turned in a half-assed performance because they were only interested in re-releasing their old games on handhelds.