There have been many developments recently on the PSP scene. The famous cracking of the PS3 has opened the way to making signed eboots for the PSP a reality. Unfortunately the signed eboots are limited to a specific size, and nobody has managed to make them work with static ELFs, which is how ScummVM is built.
Because ScummVM does things differently from other homebrews, using non-static ELFs (also known as PRXs) becomes difficult, but not impossible. With a lot of effort I could make ScummVM use PRX files instead, and therefore have ScummVM on the PSP be signed.
But is it worth it? I'm not so sure. You can run ScummVM on an official firmware using Half Byte Loader (http://wololo.net/wagic/2011/01/23/release-half-byte-loader-r113-for-ofw-6-37-and-below/), which now loads directly (without a game exploit). Switching to a PRX file really means a lot of work, and the size limit also means it's not very future-proof, as the ScummVM executable is constantly growing.
That's why for now I recommend using HBL to run ScummVM. I think it's a good enough solution, and most homebrew users have a custom firmware anyway.