tritium decays into helium by this reaction:
as you can see, the radioactive emmission is an electron. the electron excites the phosphorous, causing it to release a photon.
tritium has a half life of roughly 12 years, which means, in a situation of abundant phosphor, the tube would be glowing at half intensity 12 years after its production.
however, because the companies who make these things skimp out to maximise profits, they use low quality phosphor, and less-than-optimum amounts of tritium, which is why cheap ones (aka, NEW ones) don't glow for as long.
i feel i should mention, low-energy beta radiation (emmission of an electron remember) is completely harmless. how many of you have used a CRT monitor in your life? the electron bathing you experienced from using a CRT is many many orders of magnitude greater than having one of these an inch from your testicles. furthermore, electrons (unless travelling at reletivistic speeds) cannot penetrate skin.
Science wins, naysayers lose. awesome.