How many Boeing aircraft of each model, are currently in service worldwide?
(If additional information is available, I would appreciate knowing
the same figures for past years, and projections for future years).

Thank you for your question.
U can get anything about the boeing aircrafts
from their site www.boeing.com.
Also u'll get  lods of info from 
http://www.boeing.com/commercial/flash.html

What you are asking is near impossible to answer. As you probably
know, aircraft might go from passenger service to US cargo service to
cargo service in the 3rd world. Also, there are still WWII vintage
Boeings in firefighting service. Some aircraft that are not in
"service" still fly to Oshkosh yearly.

http://www.airliners.net/ is a good place to start. The have
production numbers for Boeing aircraft.

The FAA (www.faa.gov) also keeps a list of all domestic aircraft with
N numbers, downloadable from their website. This is US only though.

The actual Boeing website will not tell you what you want, especially
if you need info about older planes like the 707 or piston aircraft.

Chances are, a research could tackle this and give you a fairly
accurate answer. You would need to spend a lot more then 2 dollars if
you want someone to spend the few hours to find a decent answer.