I know I'm not the first person to mention this (I saw link from another blog that puts the idea of Spore and Midcore in the same sentence), but I just want to reiterate just how much SPORE seems fits the idea of "Midcore".
I'm currently playing Spore on my new iMac and it runs beautifully. My two older girls and I have been building creatures, buildings and vehicles for the past two days. The builder alone is worth the price of the software. In the past two days I have heard my daughter say the following:
"I love Spore"
"The only game we will ever need is Spore"
"Can I play Spore?"
"Hey dad, can I Spore the Spore Spore's Spore?"
I will have a full Midcore review after we have played through more of the game, but from my perspective, at the moment, Spore just might be the game that splits the hardcore from the casual and firmly solidifies the Midcore as an viable game-playing, game-buying, audience.