I love wearing jeans and a t-shirt. Would this be an issue if I want to play in a casino in Las Vegas, USA? Do some casinos have a dress code?
For the general area, there's no dress code - you even see people wandering through in their swimming gear after they've been in the pool! I've been in wearing tshirt and jeans, feeling underdressed until I saw the tourists in their tshirt and shorts with flip-flops.
However, you may wish to consider what else you want to do in Vegas, and dress accordingly. Many nightclubs have dress codes, as do some of the restaurants. Night clubs in fact often suggest business casual - leather shoes, collared shirts, and so on.
If you're seeing a show, people tend to dress up for those - smart long-sleeved shirt and trousers, although again it's not a strict rule.
And if you're just wandering the streets looking, it's handy to have light clothes in summer - it can get seriously hot!
Generally there is no strict dress code on the casino floors (within reason), so Jeans and T-Shirt would be just fine. There is a blog entry on the subject from Las Vegas Direct and also on TripAdvisor. There is also a somewhat decent FAQ regarding age restrictions.
If you get into an exclusive area where stakes are rather large you might be required to wear a suit and tie.