How likely and severe is the threat of a gamma ray burst to earth? In the National Geographic article: http://news.nationalgeographic.com/news/2009/04/090403-gamma-ray-extinction_2.html it is suggested that a gamma ray burst likely caused a mass extinction in earth's history.  How severe is the threat of a gamma ray burst to life on earth?  It would be nice to see a chart that considers the threat within the next hundred years as follows (I have no idea what the actual numbers are, hence I am asking):

Severity - Likelihood in 100 years:


Global extinction - 0.0000001%
Major impact - 0.001%
Moderate impact - 0.1%
Minor impact (e.g. deplete ozone layer by 0.01%) - 10%
Observable - 100% ("minor" gamma ray bursts are frequently detected)


Follow up questions are: 


Would there be any warning of an impending severe gamma ray burst?
Could we survive in gamma ray burst shelters? (it would appear being underground may be sufficient according to: http://en.wikipedia.org/wiki/Radiation_protection#Shielding_design and http://2013.org/showthread.php/188-How-does-one-prepare-sufficiently-for-gamma-radiation-in-a-shelter-etc)


Something else I found to be shocking is that the Bible appears to predict such an event.  I would rate the predicted event on the major impact level.  I asked about this in the Christian Stack Exchange here:
https://hermeneutics.stackexchange.com/questions/9324/interpretations-of-revelation-168-11/9335#9335
I'll address WR104 first. The National Geographic article calls it a &quot;potential threat.&quot; Yet that potential may be low. There are a slew of articles quoting astronomer Grant Hill on the subject. Hill studied the star and found that it looks like it isn't pointing straight at us. Its axis might be up to 45 degrees in another direction, meaning that we'd be fine if it underwent a burst.
From here:

It would appear the original Keck imagry may not have been as straight-forward as it seemed. Spectroscopic emission lines from the binary pair strongly suggest the system is in fact inclined 30°-40° (possibly as much as 45°) away from us.
So, Earth doesn’t appear to be in the firing line of WR 104 after all…

Here and here Hill is quoted as saying that the star looks like it could be pointed at us - which goes against the evidence he found! Finally, this is another example of Hill's cautious attitude: The evidence says we're good, but he's not making any assumptions.

Sabre Tooth, I know you said that you didn't want Wikipedia, but my primary goal here isn't to collect the bounty, so I'm going to reference it in this section. I hope you're okay with that. I may withdraw it, though, if Jonathan indicates that he doesn't want it.
Okay, back to the topic at hand. Wikipedia, of course, has a short tidbit on the frequency of gamma-ray bursts effecting Earth:

Estimating the exact rate at which GRBs occur is difficult, but for a galaxy of approximately the same size as the Milky Way, the expected rate (for long-duration GRBs) is about one burst every 100,000 to 1,000,000 years. Only a small percentage of these would be beamed towards Earth. Estimates of rate of occurrence of short-duration GRBs are even more uncertain because of the unknown degree of collimation, but are probably comparable.

'A small percentage' isn't too specific. The BBC article that is cited in this passage says the following:

Observations of deep space suggest that gamma ray-bursts are rare. They are thought to happen at the most every 10,000 years per galaxy, and at the least every million years per galaxy.

However, the 'small percentage' is never elaborated on.

That's all I have for this edit; more to come later. By the way, this Physics.SE question may interest anyone reading this. . .