I can speak from experience on this one. They most certainly will card you in the casino, they are usually very strict about this policy. Fake ID anyone?
On a side note it is very possible to count 5 decks, and even up to 8 decks. There are many different counting systems where this is more than possible. And if I had a computer with me at the table I could count any number of decks the casino wanted to throw at me.
DayvidB...how was he caught? Unfortunately on CCL they use continous shuffling machines so it is impossible to count.