I've used tarot readings in game with some success, but they were scripted rather than actual readings. I also contrasted that with a Goblin shamaness who did an animal sacrifice/entrail reading in the same game. Again no actual animals were harmed--it is a play by post game here.
I remember reading an article about 4-6 months ago involing using the tarot in character creation. Might have been in Kobold Quarterly? Anyway, I couldn't find it, but I did find this:
Character Creation (or "Smiley Face") Spread - Aeclectic Tarot Forum.
Also, here is a short article from Gnome Stew that might give you some ideas. Again, it is more about character/npc creation and idea generation than actually using the cards as a focus of the game, but you might get something useful from it.
Deep as a Puddle: Character Development with Tarot - Gnome Stew, the Game Mastering Blog
I think you could use the tarot sort of like action points. Let the players choose to draw a card to modify a roll they don't like. The nature of the card could have an effect on the result.
If that is too random, perhaps you could do a reading at the beginning of the game and let the players decide when each card comes into play. The better job they do explaining why the card relates to the current situation the more powerful the effect. They could apply negative results to opponents and positive result to themselves or their companions.
Let us know how it goes. I may want to borrow your successes on this one for my own games.