The Steam icon is provided up to a size of 48x48px — from the looks of it, this icon is straight from the Windows client. This results in a fuzzy Steam icon in many cases: the dash in Unity; the Launcher in Unity if configured to be larger than the default; essentially anywhere in GNOME Shell; the Ubuntu Software Centre (if / when the package shows a Steam icon). Linux desktop software generally assumes that application icons are available at large sizes.
Here are some screenshots:http://people.ubuntu.com/~dylanmccall/steam/beta-screenshots/small-icon/unity-1.pnghttp://people.ubuntu.com/~dylanmccall/steam/beta-screenshots/small-icon/unity-2.pnghttp://people.ubuntu.com/~dylanmccall/steam/beta-screenshots/small-icon/gnome-shell-1.pnghttp://people.ubuntu.com/~dylanmccall/steam/beta-screenshots/small-icon/gnome-shell-2.png
I think it suffices to say this small icon does not fit in at all.
I suggest borrowing icons from the Mac version of Steam, instead, up to at least 128x128. Any bigger would be lovely, too. We like pretty icons :)