I don't think anyone would confuse StormPowered with Steam once they actually visit the site. StormPowered's layout is distinctly amateur. They even offer a feature comparison between them and Steam on the main page, in case you still aren't sure which is which. Also, Steam isn't called SteamPowered. It's called Steam. It only uses steampowered in its URL.
Given these facts, I doubt Valve would even bother with a lawsuit.