Just went through this and I think I worked out a decent procedure that I’m going to try to lay out here because I’m pretty sure I’m not the only one. Plus, it probably doesn’t hurt to write it down anyways.
PROBLEM: Steam won’t start because it can’t load steamui.so
FIRST: rename ~/.steam to ~/.steam-old
rename ~/.local/share/Steam to ~/.local/share/Steam-old
SECOND: Reinstall steam (sudo pacman -Syu steam)
THIRD: Launch Steam so it can create new ~/.steam and ~/.local/share/Steam directories.
…Now, before going on to the next step, let me address manually restoring old game files…
at this point, you could attempt to move your old game files from ~/.local/share/Steam-old/userdata into ~/.local/share/Steam/userdata. However, I’ve found those files tend to be out of date and restoring from the cloud doesn’t get them back. It’s an option but, I think I found a better way.
FOURTH: Set up the new Steam install like you had the old one with respect to Beta participation and Steam Play compatibility. Restart Steam as required when you make those changes.
FIFTH: Go into ~/.local/share/Steam/ubuntu12_32 and copy steamui.so to the clipboard.
SIXTH: Drop that copy into ~/.local/share/Steam-old/ubuntu12_32 over-writing the “corrupted” steamui.so that was there.
SEVENTH: Rename ~/.steam to ~/.steam-new
Rename ~/.local/share/Steam to ~/.local/share/Steam-new
Rename ~/.steam-old to ~/.steam
Rename ~/.local/share/Steam-old to ~/.local/share/Steam
EIGHTH: Cross fingers, rub lucky rabbits foot, sacrifice live chicken, (whatever works) and try to restart Steam.
Assuming Steam starts and everything is working OK, you can now delete ~/.steam-new and ~/.local/share/Steam-new.
…anyway, this is how I was able to get Steam back up and running with all my games and saves intact.