At present it seems that VS2008 still isn't supported either in the 5.1.5 release or in the STLPort CVS repository. If someone has already done this work then it would be useful to share, if possible :)
Likewise it would be useful to know about the changes required for a VS2005 or 2008 x64 build.
It turns out that x64 support, whilst not explicitly stated, just works. If you set your environment up to use the x64 tools by running \Program Files (x86)\Microsoft Visual Studio 9.0\VC\bin\amd64\vcvarsamd64.bat then run configure.bat for your compiler and build as normal you end up with appropriate libs and dlls.
Unfortunately the x64 libs build to the same names as the x86 libs so it's not possible to have a 'side by side' installation of STLPort to allow you to build with either x86 or x64.
Edit: I've written up what you need to do to provide side-by-side x64 and x86 support as well as packaging up the changes required for vs2008 builds on my blog. See here: http://www.lenholgate.com/blog/2008/10/stlport-515-and-vs2008-and-x64.html
For other versions of Visual Studio see here: http://www.lenholgate.com/blog/2005/12/stlport-50-and-multiple-vc-versions.html, here: http://www.lenholgate.com/blog/2007/05/stlport-513-and-multiple-vc-versions.html and here: http://www.lenholgate.com/blog/2010/07/stlport-521-and-vs2010-and-x64.html