Questions and postings pertaining to the development of ImageMagick, feature enhancements, and ImageMagick internals. ImageMagick source code and algorithms are discussed here. Usage questions which are too arcane for the normal user list should also be posted here.
Your exact commandline works for me with ImageMagick 6.7.7-10 and 6.8.3-9. Maybe photoshop can't handle transparency in BMP images? Works great in gimp. According to the internet, transparency in bmp images are non-standard.
ps. This question should probably go to the user part of the forum?