Thai Visa Requirements
Before you book your trip, you'll want to check to see if a visa is required. Thailand has traditionally been rather free with visa-free entries, and most westerners still receive 30 day entry stamps on arrival. However, like just about every other country since 2001, security concerns have lead to a reduction in the number of countries automatically granted visa-free entry. For a current list of countries entitled to visa-free entry, visit the Thai Foreign Ministry's web site.
If you do need a visa, or plan to stay longer than 30 days, you can generally easily get a 60 or 90 day visa at most Thai embassies and consulates. This can usually be done by mail if there's no consulate in your city.
Note: As of March 2006, the penalties for over-staying your visa have been significantly increased. Although you are given one days grace, over-staying your visa by two days will now cost you 1,000 Baht (33.50 USD). More than two days will add 500 Baht (16.75 USD) per day. If you need to over-stay your visa due to a problem getting flights on the right day, you are urged to visit the immigration department on Soi Suan Plu in Bangkok to get a short extension to your visa. The cost of extending your visa is much less than the over-stay penalty.
While we're on the subject of visas, it's worth noting a little detail about passports as well. You may not need a visa to enter Thailand, but even for a 30 day visa, your passport needs to be valid for at least six months from the date of arrival. This is a common requirement of many countries, and it's an often overlooked detail. I have heard stories of people arriving at the airport for their flight to Thailand, only to be refused boarding because their passport didn't meet this requirement.