Can I get snow and light at the same time?

Your best bet is to place torches or glowstone (or even fire or lava) a sufficient distance in the air (using lampposts) that the light level at the ground isn't sufficient to melt the snow, but is sufficient to stop mobs spawning. The answers in this question should help you with this calculation.

If using the standard Minecraft server isn't a requirement, I know there are a ton of Bukkit plugins that let you control the melt of snow and ice (and presumably Canary has similar, although I don't use it.)

yes, you just use glowstone, add 2 layers of glass and 1 layer of leaves at the top to stop the heat, but not the light and you cover it with snow, no melting at all, just since the glowstone is kinda in the ground, it lights up a bit less, but it is invisible, so you can use it as much as you want