I can confirm that, it segfaults when writing image to a file (Xubuntu 10.10, sfml2, x86)
Here's a stack list:
#0 0x1477b7 stbi_zlib_compress(unsigned char*, int, int*, int) () (/usr/local/lib/libsfml2-graphics.so:??)
#1 0x149823 stbi_write_png_to_mem(unsigned char*, int, int, int, int, int*) () (/usr/local/lib/libsfml2-graphics.so:??)
#2 0x149ab8 stbi_write_png() (/usr/local/lib/libsfml2-graphics.so:??)
#3 0x14c540 sf::priv::ImageLoader::SaveImageToFile(std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::vector<unsigned char, std::allocator<unsigned char> > const&, unsigned int, unsigned int) () (/usr/local/lib/libsfml2-graphics.so:??)
#4 0x141284 sf::Image::SaveToFile(std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) const() (/usr/local/lib/libsfml2-graphics.so:??)
#5 0x80488bb main() (/home/nulloid/prg/bug/main.cpp:8)