Proof of God's existence by 10th century Bishop

Exactly.

Frankly, the dude couldn't have been very smart if he didn't realize that you could replace the word "God" with any word you want in that "proof" and thus establish that it exists. This type of spot checking is important, and any competent mathematician or logician is expected to have worked out the limits of any new method to see if there's potential for ridiculous results to be "proven" as a result.

The only interesting part of this is pinning down exactly where the logic fails. And even that is not very interesting, because it devolves into bickering over definitions: what exactly do we mean by "great"? Couldn't we imagine a thing whose "greatness" depends on its non-existence? ("Greatest non-existent entity" would result in some ridiculousness if you tried to apply this proof to it) What does "exist" mean? Mario and Luigi "exist" in a sense, even if they are fictional, and a good case could be made that they are, indeed, the greatest video game characters of all time (okay, maybe not Luigi :) ), despite their real world "non-existence". And so on...the end result being that we've neither proved nor illuminated anything at all, except that our original phrasing was too vague to mean anything.

But I'll give the philosophers this: they give the rest of us a great deal of evidence that you can prove very little about the actual world without actually referring to it to test your assumptions: they've been trying for several millenia, yet have accomplished all but nothing.

I think the explanation from wikipedia makes more sense (to me anyways) than the little graphic in the article:

  In Chapter 2 of The Existence of Nature and God Anselm′s
  Argument for the Existence of God is as follows:

    1. God is something of which nothing greater can be thought.
    2. God may exist in the understanding.
    3. It is greater to exist in reality and in the
       understanding than just in understanding.
    4. Therefore, God exists in reality

  Anselm's second argument

  Anselm in his Proslogion 3 made another a priori argument
  for God, this time based on the idea of necessary
  existence. He claimed that, if God is that than which no
  greater can be conceived, it is better to be necessary
  than contingent; therefore, God must be necessary. To sum
  up:

    1. God is the entity of which nothing greater
       can be thought.
    2. It is greater to be necessary than not.
    3. God must therefore be necessary.
    4. Hence, God exists necessarily.

I like to think about the whole God topic with Immanuel Kant's theories in mind (i.e. that there may be things that exist but that we cannot physically reason about).

In that light, I'd say that the difference between a startup and God (by the omnipresence definition, anyways) is that your startup must be human-made by definition, thus it cannot exist in a manner that cannot be reasoned about.