Visual Studio 2005 SP1 included new versions of some C++ libraries. Your application's manifest specifies what versions of the DLLs it wants -- so if you build with SP1, the application is going to look for SP1 versions at runtime. That has consequences for your deployment strategy, since non developers are unlike to have SP1 versions of those DLLs yet.
Nikola Dudar has gathered some questions and answers on this topic. I'll give you the questions, read his blog for the answers:
If these questions matter to you, then you have a blog post to read, don't you?
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