Mat Marcus:
Ouch, good point. I guess one has to consider _SECURE_SCL=0 as a global setting. Perhaps the boost build system might want to make this the default, given the past performance issues of MS checked iterators, etc. even in release builds. (I haven't measured the performance imapct for vc9 yet, but I did notice that it is not a debug-only switch).
I've found _SECURE_SCL less evil than claimed. It does decrease performance, but in my experience, it does so in situations where the code is already suboptimal (repeated calls to vector<>::operator[] and use of vector<T>::iterator instead of T*). On the plus side, it does catch bugs. On balance, I don't believe that we should override the default, although we probably should provide a way for the user to set it to 0 when building Boost.