For those with a TechNET/MSDN sub, the MU-files as well as the full .ISOs of Windows 8.1 Update 1 are now up!
MU update files (x86, 360 MB)
mu_windows_8.1_windows_embedded_8.1industry_update_x86_4046911.zip
MU update files (x64, 770 MB)
mu_windows_8.1_windows_server_2012r2_windows_embedded_8.1industry_update_x64_4046913.zip
ISO (x86, 2934 MB)
en_windows_8.1_with_update_x86_dvd_4065105.iso
ISO (x64 3924 MB)
en_windows_8.1_with_update_x64_dvd_4065090.iso
MU update files (x86, 360 MB)
mu_windows_8.1_windows_embedded_8.1industry_update_x86_4046911.zip
MU update files (x64, 770 MB)
mu_windows_8.1_windows_server_2012r2_windows_embedded_8.1industry_update_x64_4046913.zip
ISO (x86, 2934 MB)
en_windows_8.1_with_update_x86_dvd_4065105.iso
ISO (x64 3924 MB)
en_windows_8.1_with_update_x64_dvd_4065090.iso