music.prx or iR Shell are the best homebrew applications.
Music.prx is an updated version of the open source irsmp3.prx, and (music.prx) was previously known as mp3plugin if I recall correctly. It isn't as easy to use as iR Shell for music in games, but since it is a plug-in you don't have to boot it upon loading and it supports atrac3 files.
Does not support any custom firmware beyond 3.52 M33-4 in my testing, though unofficial versions are known to work on 3.71 M33-4.
iR Shell is a bit more complicated and may support atrac3, it has many other features and isn't as versatile hotkey wise. Has many other functions for managing your homebrew.
As for this ever becoming an official feature, that possibility is still open but unlikely for Slim PSP's since Sony could allow you to have that feature active when you don't get the added benefit of the extra RAM. But since they are allowing developers use 333MHz instead of 222MHz, it seems unlikely since it would still slow down the system for older games and could even cause them to crash or leave it vulnerable to new software exploits.
