I've replaced a couple of 3GS screens which is very similar.
It can be done and my advice would be to get the screen & digitiser in one unit rather than just the glass screen. It's very fiddly to remove the shattered glass otherwise and you may end up damaging the LCD itself.
It's about £20 for the entire unit from ebay complete with tools to open up the unit.
Then you can go to Youtube and find lots of videos/tutorials on how to do the replacement. It's not actually that hard but the first time you do it of course you will be a bit slow.
But when all said and done you may want to call around some local independent phone shops which can take the hassle for you. A friend of mine was charged £35 for her repair which given the cost of the parts isn't a bad deal for someone to take the risk away.
Anyone done it? I dropped mine, it still works but the screen is shattered. Its an old iphone 3G which I love, and is ancient so not under any warranty. I have seen kits to do this but wondered if anyone had tried the do it yourself kits, or any other solutions???