I'm pretty sure it didn't accept my key though (or I couldn't read it properly, it was quite worn out, being a sticker pasted outside the jewel case).
I beg to differ.
Eons ago, when I played (OOh the memories) Starcraft I & Expansion on a heterogeneous network my Macs accepted Windows Keys for the installation and vice versa.
If I remember correctly Starcraft even accepted the most glorious of keys: 12456789012 as a valid serial number.
So go my little celebrate, kill some Protoss heathens.
No, no. I may remember wrongly (it was a while ago, after all), but what I meant was that battle.net refused to accept the key for my old Starcraft copy (or, as I said, I couldn't correctly discern what the key was as the sticker couldn't be read and I may have copied it incorrectly). Instead I got a new one (they were cheap back then, I've no idea of the common price now), and registered that.
As a bonus, I have a version for classic Mac OS (on disc), something you don't get off battle.net (just OS X).