What immunisations, if any, are required / recommended for Thailand?

According to the Thailand entry on the Finnish site rokote.fi,

These vaccines are recommended for everyone:

• Hepatitis A
• DPT / DTwP
• MMR

These are recommended for some, based on a risk estimate (preferably conducted with a health professional):

• Hepatitis B
• Japanese encephalitis (but see also Ginamin's answer!)
• Cholera
• Typhoid

Preventive malaria medicine is also recommended for parts of the country (green in this not very detailed map). Seems like Bangkok and the coastal areas most frequented by tourists are not in the higher-risk zone though.

The site cites this source for their vaccine information:
Matkailijan terveysopas 2009, a publication by the National Institute for Health and Welfare

Nothing, really, unless you mean to stay in the jungles for a while. And yes of course you want mosquito repellent because skitos are never any fun.

Your biggest health concerns have to do with heat, hydration, food and traffic accidents. If you drink 2 liters at home, you need 3 liters in Thailand (assuming of course a temperate climate where you are from). You also need to drink electrolytes regularly.

Good sunscreen. Don't laugh, this is not your typical "hey, I got sunburned", mess around with that and you could spend the rest of the holidays in a really bad condition.

For the food, keep in mind that tap water is not drinkable. Actually it's not drinkable in most of southern Asia, so buy bottled water only. You don't want to drink anything with crushed ice but ice cubes are usually fine. Thai food tends to be very spicy and you are likely to get mild diarrhea (which brings you back to the hydration issue - Diarrhea causes a loss of water & electrolytes).

Other than that, of course you have all kinds of diseases, but not more than in other civilized countries. Hygiene standards are rather decent in most places.

And traffic accidents. It's not a disease, but it's still an epidemic and as a tourist you are far more likely to suffer significant health problems from the poor driving standards than you are to catch any major disease in Thailand. (By the way, people are supposed to drive on the left, here. Let's be fair, that's also a reason why tourists have a lot of accidents).

Found this from the World Health Organisation :

Yellow Fever Country requirement: a yellow fever vaccination certificate is required from travellers over 9 months of age arriving from countries with risk of yellow fever transmission.

Yellow fever vaccine recommendation: no

Malaria Malaria: Malaria risk exists throughout the year in rural, especially forested and hilly, areas of the whole country, mainly towards the international borders, including the southernmost provinces. There is no risk in cities (e.g. Bangkok, Chiang Mai city, Pattaya), Samui island and the main tourist resorts of Phuket island. However, there is a risk in some other areas and islands. P. falciparum resistant to chloroquine and sulfadoxine–pyrimethamine reported. Resistance to mefloquine and to quinine reported from areas near the borders with Cambodia and Myanmar. Human P. knowlesi infection reported.

Recommended prevention in risk areas: Mosquito bite prevention only; Recommended prevention in areas near Cambodia and Myanmar borders: Mosquito bite prevention plus atovaquone–proguanil, doxycycline or mefloquine chemoprophylaxis (select according to reported resistance pattern)

If you come from: Angola Argentina Benin Bolivia Brazil Burknan Faso Burundi Cameron Central Africa Chad Columbia Congo Cote d'Ivoire Ecuador Equatorial Guin Ethiopia French-Guiana Gabon Gambia Ghana Guinea Guyana Guinea Bissau Kenya Liberia Mali Mauritania Niger Negeria Panama Peru Paraguay Rwanda Senegal Sao Tome & Principe Sierra Leone Somalia Sudan Suriname Tanzania Togo Trinida & Tobago Uganda Venezuela Zaire, you need an evidence of vaccination for yellow fever (Thailand immigration) in order to be allower to enter the country.

Risk of malaria is really low and malaria pills has bad side effects.