All known cases that went to the hospital, half died. What about the ones that didn't go to the hospital, is what I wonder.
You and just about every epidemiologist on the planet. It's not very difficult to determine whether a person has been infected with the virus: test for antibodies. It obviously isn't practical to do that for seven billion people, however, so what you do instead is go to places where the virus is known to have appeared, round up some of the people you figure are most likely to have been exposed, and test them. Not nearly as much of this has been done as everybody would like, but the studies that have been done are not very supportive of the idea that there is a lot of subclinical infection out there. Here's one sample:
"Participants in this study were from villages in central and northern Thailand where widespread, confirmed outbreaks of influenza (H5N1) infection in poultry and at least 1 human influenza (H5N1) case had occurred during 2004. A substantial proportion of participants reported exposure to backyard poultry, including contact with sick or dead chickens, the primary risk factor for influenza (H5N1) infection (8,9). Nevertheless, we found no serologic evidence of mild or subclinical influenza (H5N1) infection, suggesting that clade 1 influenza virus A (H5N1) strains circulating in Thailand among backyard poultry during 2004 did not transmit easily to our study population."
http://wwwnc.cdc.gov/eid/article/15/5/08-0316_article.htm
And more here, including a chart:
http://www.fluwiki.info/pmwiki.php?n=Science.Seroprevalence
One of those is an outlier: the Hong Kong study casebro is probably thinking of.
Of course, we're still talking about a virus that hasn't adapted to efficient transmission between humans, so even if we knew exactly what the current mortality rate is (that is, if there were NO uncertainty about the numbers of subclinical/asymptomatic cases) there'd be no way to know how much that would change when the virus aquired whatever characteristics would enable it to make the jump. It will be a different virus at that point.
What they've done with the ferrets seems... daring. It's easy to picture a scenario like:
"So how are Bob and those guys coming along with the ferrets?"
"I dunno, haven't heard from them in a couple of weeks, and they aren't answering the phones..."