Quake 3 came out for Dreamcast
DC only had one thumbstick, which made a huge difference.
in a shooter, that could be a non issue mostly, but in a game like portal, where you have to aim while flying and spinning, killing or stranding your teammate if you fail, it could be a deal breaker. further gaming divide?
If Portal is multiplatform I'm assuming it's designed with a controller in mind and the puzzles are created so that your average PS3 user can achieve success.
Shooters are much worse. On consoles everyone is equally limited. Throw PCs into the mix, with people able to turn instantaneously and line shots up in the same time and it falls apart. The people making Huxley struggled with this before giving up (for other reasons.) The general idea is to give console users assists but no one likes that.
The real question is "why?" What does anyone get from this? In general populations have been more than solid enough to support themselves. Sure, there are some people out there that have a 360 while their friends have a PS3 but that number is dwarved considerably by the amount of people that would run around trash talking how inferior other people's platform choices are.
Which is the main reason why crossplatform will typically be limited to private matches. By doing that you're guaranteeing no more than a handful of people actually use it. Suddenly it's not worth the considerable effort.