I always carry mine with me.

I was asked in Target the other day to show ID because I was buying wine. I told the cashier I was flattered.....I'm the wrong side of 50 [msnwink]

I read an article the other day that suggested taking a photocopy of your passport and always carrying that with you separately, just in case the passport gets lost/stolen, as it would make it an awful lot easier to get a replacement.