Go Cindy! While it's a drag she won't see a dime of that, it's nice to see living artists commanding big prices so they can actually benefit in their lifetime from their own future sales. Plus Cindy Sherman is freakin' awesome.
Cindy Sherman Print Sells For $3.9 Million At Auction, The Highest Ever For A Photograph At last night's auction at Christie's, Cindy Sherman's 'Untitled #96' blew past all previous photography records
at the risk of sounding ignorant, what's the big deal about it?
gofuckyourself.com/?g=you+twat 
XO
Let's put it this way - your question coiuld be taken one of two ways - 1. Who is Cindy Sherman, and why is she so iimportant? or 2. What makes this photo significant to the extent that it is worth worth almost 4 million dollars?
Since the article I linked to gives the author and title of the piece, it shouldn't be that hard to look up, right? Cindy Sherman, wikikpedia. Much better explanation than I could give off the top off my head, right? Without any foreknowledge though, Cindy Sherman can be safely assumed to be one of the most important contemporary photographers around, and this piece is representational of a period of her work that is considered significant enough to be of major interest by collectors. Why 4 million? Well, that's the vagaries of the art market for you. Many books and articles have been written on this subject by people who know a lot and care a lot more than I do, and the internet is a tool with which you can discover more about this topic to whatever depth you desire.
I'm not sure what the point of your question was other than to say "I can't be arsed to Google it", hence my answer.
yes I could have googled it, no I wasn't that bothered. The price tag piqued my curiosity enough to ask, like you would in a friendly conversation I didn't think these huge prices were usually on the actual merits of the piece, but more due to some curious circumstance, like who's owned it before or something.
Since it seems to be just down to her being considered awesome and the piece being appreciated for what it is, and her still being alive to cap it all, then yes I agree with you, it is awesome.
The short version of the money thing as far as I understand it is that Christie's & other auction houses have been slowly inflating the photo market, partially a spillover to inflate the value of other works from that period like Warhol, say. This has been generating a lot of interest around late 70s / early 80's work in particular - pretty much Sherman's heyday. The photo that just sold for 4 million is a very limited edition and is typical of her (IMO) best period, and is very representational of the work for which she is best known. When her last big retrospective show came to Canada, this was the image used on the poster.
Anyway, here's a good article on why her work is of such interest right now:http://www.photography-collection.com/photographers/the-return-of-cindy-sherman/
would it have killed them to throw a photo in to that article?
So it's not like she's been a poor, unappreciated artist who's only just struck gold? Sounds like she's been doing alright.
Yeah, she's been a big deal for like, 30 years.
This is my favourite series of her stuff: Untitled film Stills 1977-1982
Basically, she dresses up & does self portraits as non-specific other people in non-specific situations. Implied narrative, the "other" reversed within the context of self-portraiture, the implicit gaze of the viewer-as-self, and all that.