fopen fails of files larger than 2^31 bytes on 32-bit Linux systems. This is a known issue in the gnu library, which offers fopen64 as a fix. See http://www.gnu.org/software/libtool/manual/libc/Opening-Streams.html.
I think I will simply have phobos' fopen forward to fopen64 on linux. Are there any reasons for not doing so?