Unlike buying a PC, or a Mac with a service plan from a place like Best Buy, it is difficult to find spill protectio n (Best Buy will even cover the battery unlike AC) If Apple included that then...
Your point about others paying for it is a fair one - if and only if they actually do the repair. If there is an obvious stain it foolish to try but if it was a few drops of water then maybe it would be unnoticeable. The cost they would charge to fix will likely be mostly profit, not cost, and Apple does do very well on their macs since they never drop prices as their costs drop over the life of a product. With 30 billion in cash reserves and obscene profits on older, unrefreshed lines like the MP and MA, it could be considered payback - or you paid for it with the Apple tax. Also, another way to look at it is by fixing this little thing they earn your loyalty and you tell all your PC friends to switch to mac for their great service. I am not advocating telling a lie, (I said fein ignorance - ask me no questions I tell you no lies) just bring it in and state the problem - but don't look guilty.
Anyway, let us know what you decide and how it works out. And keep liquids far away from your computers!!!