For many of you (not many of everyone, if you look at the sales charts), Spore's iron-fisted DRM was a big turn-off. Well, after promises, now EA have delivered, freeing up the game's installs.
PC owners (sorry Macs!) can now download a tool from EA that will let you de-authorise your PC. This effectively allows you to override the game's installed-on-five-PCs limit, as you can manage the availability of all five "machine slots".
So if you hit the limit of five, just use the tool to de-authorise one machine and it'll free up a slot. Easy.