About 1.09 million - you can see how it changes over time here; http://klmist.com/keysreport.html

IMO the actual number of users is probably lower than this.. most users have multiple keys. There must be enormous overlap with all the different bitcoin wallet services and people create multiple coinbase and blockchain.info/mywallet accounts for different phones/tablets.. in the case of blockchain.info, perhaps installing/uninstalling on 3 or 4 devices could end up creating more than 10 accounts for a single user.

Also people sign up with every new service just to check it out, even if they can't use it because they live in another country.. (eg, coinbase and chinese exchanges). It's possible that there are a lot of users hiding behind large shared wallet keys, but I'm not really convinced of that.. surely anyone putting money into bitcoin will at least have one 0.01BTC key of their own too?