The Kingdom of Siam was renamed to Thailand under the authoritan regime of Field Marshal Phibun in 1939, and after a short interlude from 1945 to 1949 with a reversion the old name only the name Thailand was used. However, this name is a bit controversial as it refers to the main population group only. But ethnic Thai are by far not the only citizen of the country, there are lots of Chinese, the Malay in the troubled southern provinces, hill tribes, the Isan people who sometimes prefer to call themself Lao to show the cultural differences with the central Thai.
A great article on the name change is "Siam or Thailand: what's in a name?" by B.J. Terwiel, originally published in Bangkok Post of January 10 2008. It was written as at that time it had a bit of discussion on the name after Charnvit Kasetsiri, an historian and former rector of Thammasat University, started a petition to revert to the historical name.
Charnvit has now restarted his petition, responding to the invitation of Prime Minister Abhisit to submit proposals for amending the 2007 constitution - one of his attempts the dampen the tensions between red and yellow shirts after the Songkran riots. An English translation can be found at Prachatai (Original in Thai).
Even more interesting for this blog is the third point in his petition, where he calls for the recreation of Thonburi province. This province was abolished and merged with Phra Nakhon province to the special administrative area of Bangkok metropolis. His reason for this is simply that the administrative act was done after the 1971 coup in a undemocratic way. But with this reasoning quite a lot of things would have to be reverted, as for most of the times since the end of absolute monarchy Thailand was no real democratic country. Besides, as Thonburi and Bangkok have grown together so much already it'd make no sense to split it. On the contrary, it might even make more sense to add Samut Prakan and Nonthaburi into the city as well.