How can I disable creepers' block damage on my Minecraft server?

It's now built into 1.4.5 (and perhaps earlier 1.4.x). This is a command built into the server, and also single-player, if you have cheats enabled.

/gamerule mobGriefing false

Exactly as you want: creepers damage players but not blocks.

As a bonus, I just installed the "Ice Cream Sandwich Creeper Mod" -- in snow biome, creeper explosions leave ... well, just as the name implies. There's also full crafting support.

With WorldGuard you can!

Download this Bukkit Mod and install it

Then in the config.yml file

change the line with

block-creeper-block-damage = true

to

block-creeper-block-damage = false

With this no damage will be made to your blocks but it will to players :D

The CreeperHeal mod for bukkit enables creeper explosions with a twist: the blocks grow back over time (within 30 seconds, usually).