What are the various open source and commercial C++ libraries for probabilistic scientific calculations ? I'm mainly interested in Bayesian learning ; something similar to the Bayes Net toolbox in Matlab
GNU Scientific library (GSL) probably might give you the building blocks for what you need.
It's not a C++ library per say, but you can call R from C++ by using Rinside. So far as any calculations related to probability and statistics are concerned, you can't beat R.
Since you're interested in Bayesian learning, you will want to look through the Bayesian view on CRAN. It also integrates with other bayesian tools like Bugs (for example).
GNU R actually has a standalone library (with C bindings) you could link to as well. The documentaton is a little hidden but Section 6.7 of R Extensions is a start.
Edit: Better documentation on the standalone R Math library is in Section 9 of the R Admin manual.
You may want to check this:
http://sites.google.com/site/erwannrogard/cpp-statistics
It's heavily based on Boost, and parts of it may one day end up there.