I have mentioned before how the '3 9' we keep seeing associated with Vocaloid Miku Hatsune is a play on words that easily reminds Japanese folk how to say "thank you" to English speakers. This graphic of the Japanese numerals shows that '3 9' is spoken "san kyu" – which is near enough that we Brits (and Americans, Canadians, Australians etc) will understand when, say, a tourist asks us to take a photograph of him/her in front of a London landmark.
Now, here comes the twist, which I have left until now as I didn't wish to make life too complicated all at once. I thought it was best to get my readers into the swing of all this gradually, over a period of time.
Anyway, there are variations of pronunciation of these numerals – one of which is shown on the linked graphic, as it's so commonplace. In that instance, it's the optional shi and yon for the number four. Less common, though accepted as perfectly normal I gather, are Mi-tsu (or Mi for short) for the number three, and ku (rather than kyu, though very similar) for nine.
Thus '3 9' can actually be said as Miku! I bet you weren't expecting that...