Heh, we do the same. I appreciate that Changi airport has bottle-filling fountains at every gate.
I really appreciate Haneda airport for having bottle scanners, so you can just bring your filled bottles through security. Saw this at an airport in Europe, too, but can't remember where (domestic Athens maybe?).
Cadbury's Marvellous Creations range has been doing it for about 10 years, but I've only seen them in Australia and SE Asia (assumigly sold in UK, too).