An authenticated 4th century document is a great find.
No sarcasm in this comment--it's a really cool thing.
The problem comes when National Geographic tries to expound upon the religious implications of their find. It's then that you get statements like "The Gospel provides an alternative view of the Jesus-Judas relationship and evidences the diverse theological beliefs that circulated among early Christians."
Let's break that down, shall we?
an alternative view of the Jesus-Judas relationship
Can someone say Da Vinci Code? Look, I like conspiracy theories as much as the next guy, but you gotta have at least a shred of credibility to pull them off. The real conspiracy here may be that the Society is gonna charge people 22 bucks for a hardbound edition of this text.
evidences the diverse theological beliefs
This is news? Look, I don't expect anyone from National Geographic to read the Bible or take it seriously, but you readers should, and when you do, it won't take long to see that this was an issue even while the Bible was still being written. See Gal 1:6-9 and 1 John 2:18-26 for just a couple of examples of this (from the top of my head). There are plenty more, I'm sure.
that circulated among early Christians
Can the folks who believed in the Gospel of Judas be considered Christians? I don't think so. They were Gnostics, meaning that they believed in "secret knowledge" that was only available to a select few individuals. The true path to salvation was hidden to all but a select few. (Some think Calvinism teaches the same thing, but we'll save that discussion for another post). The Bible, by contrast, presents a picture that the all the information is there, but for whatever reason (depending on your view of depravity, free-will, etc.), men don't accept it. However you slice it, the way to God is found in the Bible; it's all there--there's no need for hidden texts or special revelation (back, you rabid Calvinists! A new heart is a separate issue from what I'm talking about here).
Here's a little taste what you'll find. Let me warn you, though, don't read this with others around--your laughter might be so loud that it could turn into a CLM! (Career Limiting Move--for those of you who have never been a Schwabbie.)
SCENE 1: Jesus dialogues with his disciples: The prayer of thanksgiving or the eucharist
One day he was with his disciples in Judea, and he found them gathered together and seated in pious observance. When he [approached] his disciples,  gathered together and seated and offering a prayer of thanksgiving over the bread, [he] laughed.
The disciples said to [him], “Master, why are you laughing at [our] prayer of thanksgiving? We have done what is right.”
He answered and said to them, “I am not laughing at you.
are not doing this because of your own will but because it is through this that your god [will be] praised.”
They said, “Master, you are […] the son of our god.”
Jesus said to them, “How do you know me? Truly [I] say to you, no generation of the people that are among you will know me.”
THE DISCIPLES BECOME ANGRY
When his disciples heard this, they started getting angry and infuriated and began blaspheming against him in their hearts.
When Jesus observed their lack of [understanding, he said] to them, “Why has this agitation led you to anger? Your god who is within you and […]  have provoked you to anger [within] your souls. [Let] any one of you who is [strong enough] among human beings bring out the perfect human and stand before my face.”
They all said, “We have the strength.”
But their spirits did not dare to stand before [him], except for Judas Iscariot. He was able to stand before him, but he could not look him in the eyes, and he turned his face away.
Judas [said] to him, “I know who you are and where you have come from. You are from the immortal realm of Barbelo. And I am not worthy to utter the name of the one who has sent you.”
Oh, Barbelo. Now I get it (yes, that is sarcasm).
Don't take my word for it, though. Read the whole thing online (or at least as much as is out there--at one point 17 lines are missing). You can also Download the download a PDF copy from National Geographic, or read it from their page (cool Flash effects--wish I could do something like this on my page!).
So what's the bottom line? If anything, it makes Christianity's claim stronger. It's almost unfair to put the four Biblical gospels up against something like this--it would be like my five-year old playing Michael Jordan in basketball, or me trying to beat Bugblaster at chess--it's just sad.
As far as responding to those around you, I can't do better than quote James White here:
So should someone come up to you at work going, "Hey, Bob, I know you are a Christian, but how about that Gospel of Judas! Sure throws your Bible into a tailspin, doesn't it?" just smile and respond, "Hey, I heard about that. I've been wondering all morning how a work of fiction written more than a century after the fact by a writer seeking to promote a completely different religion than that of Christ and the Apostles that doesn't have a shred of historical foundation to stand on could possibly get so much major air time.
Gospel of Judas links
National Geographic links
Explore the Document (Can't say enough about the awesome graphics!)
Download the English translation of the text (in PDF).
Layman's guide to Gospel of Judas from Christian CADRE. (HT: Peter Chan).
James White's take. (White echoes my thoughts by encouraging to read it for yourself.)
Pastor Rod has a nice writeup plus a roundup of additional links (Rod & I disagree about lots of stuff, but I'm happy to stand with him on this; I'm also interested to see his answer to the comment "How do we know the real Bible is true and the Gospel of Judas isn't?" People you know and some you work with probably have that same question, so take note).
James Swan wins the award for funniest comment on the Gospel of Judas.