The article names here have gone back and forth between 'Ko' and 'Koh' a few times already. While I generally advocate sticking to the Royal Thai General System, here I'd cast my vote for Koh due to the following reasons:
More popular. "Koh Samui" gets 1,330,000 Google hits, "Ko Samui" gets 303,000.
Easier to pronounce. It's a long o, not something that rhymes with "Cow".
I don't particularly mind -- it seems that every "Koh" article gets moved to "Ko" so I've been updating the links. In either case we probably should have redirects for the lot of them. ("Cow" has a dipthong, not a short vowel, at least in Australian English -- "Ko" to me reads more like it has the 'o' vowel in "box".) -- Hypatia 22:26, 28 Nov 2004 (EST)
For the record, I changed my mind, and the Wikitravel standard has for several years been Ko. Jpatokal 09:46, 27 August 2007 (EDT)