Google PCs is probably the greatest risk IMO. Just as MS is trying to get people interested in Windows mobile via using their PC ecosystem, Google could wave PC's "like you phone" in front of people promising simplicity and reliability (true or not) that runs all your mobile software. Although it really only matters for games as far as game developers are concerned, and it's unlikely MS will be displaced from that position any decade soon even if they completely lose the casual PC market. If tablets start to get real software, such as real image editing that can load your RAW images from your camera and tweak them, or design and print a decent document, I can believe MS will start to lose marketshare.
Valve kinda have the same issue as Google. For real software, Windows or OSX is necessary. Unless you can get the productivity apps on your OS, it'll be marginalised.