I think it's very likely that Maxis will release official rules for the trading card game. Will Wright is too smart to not do that.
I've always like trading card games, but I never really got into them because you have to waste tons of money on buying them. Yes, games are fun, but spending so much money on a single game just to have a tiny bit more variety in your deck is just plain stupid. It's worked so far because little kids are too stupid to realize that all that money could have bought them a ton of different games rather than wasted on a single one that they'll eventually grow out of and then they just have a pile of painted pieces of paper that are good for nothing.
And that's exactly why I'm very exited about the Spore trading card game, because you don't have to buy the cards! Sure, Maxis might sell official prints, but you won't have to buy them. Even if they don't include a feature in the game to print them out, you can simply take a screenshot and print them out yourself (if you really really wanted physical cards). Will Wright's not an idiot, he knows there's no way to stop people from printing their own cards, and I doubt he would even want to. The whole point of this game is about expressing your creativity, so I think it's likely that they'll include a printing option built into the game. But, you'll definitely be able to download them into your cell phone and most likely some other mobile gaming device, like a DS or a PSP, because that'll allow you to play the card game electronically. Personally, I'd much prefer that format, and I guarantee you most people will.
And if there's people out there who for some reason believe that throwing their money away on cards is the best way to spend their money, then by all means, buy the official prints (if they do make them).
However, a physical card game version will be very limited compared to the electronic version. In case you didn't notice, there's going to be millions of planets that you can visit in Spore, which you could never finish in your entire lifetime. That means there's an endless supply of cards. How exactly do you expect to carry around a deck of 10,000 cards or more? With a handheld device, it's no problem.