Wiz and Pete are spot on so far as the current model is concerned.
The reason the 75 will be around for a while however is a few of these things, payload, performance, and crucially lease costs. If you look at the lease rate for a newish 737 NG (ala 800 etc) compared to a 15 year old 757, the 757 is just under half the money, add in 30-40 extra revenue seats and the previously mentioned points, I'd say its got 7-10 years of life left in it in pax service in western countries.
Have read and heard some interesting noises coming out of Boeing regarding a possible narrow-body replacement for the trusty 75. There are already murmurings about the limitations of the 737 in its 900 stretched incarnation as it is considered a stopgap by many.
Boeing give the impression to me that a narrow body likeness of the 787 is being studied, and as Airbus have nothing between the 321 and 330, there is a gap in the market place, whether that translates into aircraft orders is an entirely different question.