Operating systems are platforms which unify distinct hardware under the one umbrella. What the user does with the system is up to the user. For example it seems that more people buy the PSP for media playback than gaming based off attach rates. Does that mean that the PSP doesn't count as a platform for gaming? In addition to this, as future consoles do far more than gaming as well then how do you categorise them?
The iTouch, Phone and soon Pad are the same ecosystem even though they are three distinct pieces of hardware. People have no trouble describing them as a unified platform and they have no problem with the idea that part of their appeal is the mobile games and apps you can get on all three. Windows mobile 7 is also fairly unified in terms of the hardware basis for the phones even if the manufacturers are distinctly different.
In the home operating system space, Microsoft seems to have spent considerable time and money developing their gaming related tools and APIs. Infact both the Xbox 360 and PS3 bear Microsofts influence in the design of their respective GPUs. Its no happy accident that a wide range of gaming does happen on the Microsoft system and people are aware that if they wish to play games they have to go Windows. How does this not add to their overall gaming 'presence'?
There are some differences between Windows and console platforms and, I believe, iTouch and iPhone. MS doesn't get any money from Windows games because there are no license fees for games. And MS has no control over who publishes games for Windows because anyone can release and sell games for Windows without permission from MS. MS only gets money from development tools, API's and OS sales. It doesn't matter if game would not even be released or it would sell 100 million copies or only pirates would download the game because MS would not get a penny from game sales. All money to MS comes indirectly from other sources. I am assuming this also applies to Windows mobile (could be wrong though). Only games from which MS gets money directly are games which MS has published and/or developed.