PlayStation 3 developer ps3hen over at PS3News has released a hack that allows installing
retail PKG's on firmware 4.0. Although you cannot run homebrew or unsigned code it serves
a purpose to those with hardware flashers using dual boot and who knows what other
possibilities.

This is hack does the following things:

adds "Install Package Files" and "/app_home/PS3_GAME/" to "GAME" on the
XMB (allowing the user to install retail package files anytime they want)

adds other debug functions which are small but still there
does not give access to "Debug Setting" (reasons for this are explained below)

This installation procedure is the similar to technodon's original dev_flash hdd swap
procedure.
For this you will need two hard drives a e3 flasher or similar device