Once the Allegro 4.0 was stable the development team decided to redesign the API because it was originally designed for old Atari ST systems and then ported to MS-DOS. The new API will not be compatible with the current one but will fit better with newer operating systems as Windows, GNU/Linux, MacOS and so. This new API will be released with Allegro 5.0 but the development team are testing it in version 4.3.
Also, the current API is C-style, using undersores and lowercases, as in al_set_config_file and AL_BITMAP. The Allegro.pas API would be more Pascal-style, as in alSetConfigFile and TalBitmap. I'm not sure about this.