I think they do take bets on almost anything, though there are certain limits. For example I think it's difficult to place bets on your own life expectancy, except under very specific circumstances: I recall someone did it last year. (You can, however, bet on whether you live to a certain age, e.g.
http://news.bbc.co.uk/1/hi/england/surrey/6586637.stm)
However, most wagers placed are for very specific events: City scoring three in the first half against United, Smith winning the Cup on Old Nag, Jones running 100m in less than 9 seconds before the end of 2010,
whether cousin Chris will compete for his country before he's 30, etc.
To go in and ask for five or six pretty detailed plot points would be suspicious, despite many other fans would be able to predict similar ideas. As I've espoused in other threads, I had a dozen or so questions that I was expecting to have answered by Hallows. I could have wagered on the outcome of those and would have won on at least half, though some weren't sufficiently yes/no for a bookie (e.g. was it love that saved the day: I'd say no, although it helped in at least three major places, it was wandlore and ownership that ultimately let Harry beat Voldemort, so no win. Okay, I guess they all do come down to yes/no.)