I'm not going to bother getting into the specific philosophical definitions here, because it's not philosophical agnostics I hate. Philosophical agnostics are awesome. It's the colloquial ones I'm not so fond of:
1. A person who, either intentionally or purely out of apathy, hasn't put an ounce of thought into the question: "Does God exist?"
2. A person who is desperately afraid of arguing with people, presumably because he or she has a bunch of fundamentalist Christian friends and prefers to play nice with them instead of forming his or her own opinions. Paradoxically, he or she will argue at length about how arguing is ultimately "pointless."
3. A sadly misinformed person who thinks "religion" just means "something people talk passionately about," and is infuriatingly quick to dismiss anything and everything he or she disagrees with by waving his or her hand, scoffing, and saying, "You're just like all the other religions."
Addressing them in order, I'm totally okay if you haven't put that much thought into the question. It's much more productive to put your thought into getting a college degree or something, and college degrees have the added bonus of actually existing. Of course, every God humans have worshiped usually comes with a few requirements about how you should live your life, so you should probably give the idea a bit of thought at some point.
To you socialites who prefer to have a lot of friends over a monolithic philosophical orientation, good for you! Just don't try to tell me I'm "wrong" to argue that your friends are worshiping a made-up God. There's no evidence to suggest that any of the currently worshiped Gods actually exist, which is all the more reason to think they're invented. If you're afraid your social standing will be affected by my arguing, find yourself a restroom to hide in until I'm done.
... or you could actually figure out what you think. It doesn't hurt that much, really.
And to all you who fall into the third definition, buy a dictionary and fuck off. Maybe you could throw some money away and sign up for one of those Scientology seminars for a weekend. Roll with the Scientologists (or the Mormons, or Baptists, or anyone, really) for a month or so and see if you still think the "Atheism is a religion" argument is tenable.
Why the Agnostic position doesn't work
If you're one of those who is quite passionate about how we can't know for certain whether or not God exists, and who is adamant about how the best approach is to hold neither opinion, I offer you this question:
If I tell you that there is a group of giant, invisible fairies with bat wings behind me, ready to curse you if you don't give me $20, how do you justify any action at all? If you give me the $20, you're lending undue credence to a claim that's unverifiable. If you don't give me the money, you're also lending undue credence to a claim that's unverifiable.
If, as you say, it's completely irrational to hold any opinion about things which aren't 100% provable, how do you justify any action at all? Why go to work, when you can't be 100% sure your boss even exists? Why pay your taxes, when you can't be 100% sure the IRS exists? Why do anything, when your entire understanding of the world is given to you by our fallible senses?
The much more rational approach is to weigh probabilities. Your boss hands you a check every two weeks, and he/she keeps showing up at your workplace, criticizing or commending you. How big of a leap of faith are you making when you say, "I believe my boss exists?"
Not a very large one at all.
So, then, how large of a leap is it to say, "This God exists," compared to "This God doesn't exist"? When the evidence for a given God is all about miracles and supernatural events, and every single one of them can be explained naturally, the leap to "this God doesn't exist" is much, much smaller than the alternative theistic position.
Additional evidence makes the required leap of faith for some claims larger ("There are Giant Bat-Fairies behind you") while it makes others smaller ("You're just making that shit up").
So, which requires the bigger leap? Does God exist, or does He not?