While Ubuntu is not my personal cup of tea and as such I have no interest in actually using it I am thankful for it's existance as it has indeed improved Linux desktop share and has become the closest thing to a 'Linux for the mainstream' solution.
Also barring the existance of a 'linux powered steam-box project' I'd wager a company like Valve would never had targeted Linux for Steam if it haven't been for a mainstream desktop end-user oriented distro like Ubuntu, no matter if Microsoft was threatening to eat their lunch or not.
Of course the downside with an emerging 'standard' desktop distro is that it can cause incompabilities when software is targeted directly at a specific distro, particularly if that distro developes 'non-standard' subsystems which are hard to support elsewhere unless you are willing to 'buy the whole farm'. The latter is certainly a risk as it seems Ubuntu is trying hard to have developers target their distro specifically rather than Linux distros in general.
Overall though I think Ubuntu has had a great positive effect on Linux on the desktop, and also that if they start overstepping boundaries there will be an appropriate reaction (like with the amazon search 'feature'). When all is said and done, vendor lock-in isn't really an option when it comes to open source systems.