Question: What i'm wondering is, what kind of behaviour does google analytics show when a ddos attack occurs? Any theories?

Answer: 
It depends on the way that the DDOS is implemented. If it's simply an executable distributed to multiple machines, making simple HTTP queries using native TCP sockets, then Google Analytics wouldn't notice anything at all: because the JavaScript that gets returned would never be executed.

However, other sorts of DDOS attacks could leverage actual browsers distributed across many machines. For instance, if you could hack the Yahoo home page and insert an `<iframe src='takemedown.com'>` into it, you could easily DDOS "takemedown.com". In this particular scenario, GA would certainly detect the impressions, and because (depending on the scenario) there might be an HTTP referrer tag, you could possibly run a report in GA that could pull out the suspicious impressions.

But there are other similar scenarios that wouldn't leave any particular footprints. For instance, if you could hack Lady Gaga's twitter account, you could send out a link to her 16MM followers, and a significant number would immediately click on it: and since most of those clicking on it would probably be doing so from within a separate app, there wouldn't be any referrer tag, and no particular way of identifying the requests.

In other words, it all depends, but it's probably not a terribly useful avenue to investigate. In many (most?) scenarios, GA wouldn't even recognize the impression; and in many others, wouldn't have any reasonable way of picking out the good impressions from the bad.
Well, I'm also searching this kind of information, but I have some considerations about the answer:

You will probably not see the attack itself with Google analytics, but you should see the results, I mean, a DDoS is a "distributed deny of service", so, if the service is effectively denied, then you should see a flat line on the graph on Google analytics.