Interesting. It seems the newest firmware for PSP is not required to access PSN so there is little reason to upgrade to it. From what I've read it fixes known exploits so ISO loaders and that HEN thing doesn't work, but apparently signed homebrews still work on 6.37.
I downloaded 6.35 the other day, so I'm going to give it a try.
EDIT
Heh, have to charge my PSP. Even just sitting there the battery goes dead, no idea why.
Note that I'm only doing this because I have a Pandora battery and can go back to custom firmware all I want. If you have CFW and no pandora, might want to see how this all pans out before goofing with it.
It seems you cant sign things over 4MB, which works for homebrew but wont let you "sign" ISOs. Looks like an optimal situation for Sony.
UPDATE
So I installed the 6.35 official firmware. Cant seem to get on PSN since it tells me theres an updated firmware. It'll be a bit before I get the latest downloaded. But on official firmware I was able to use the Wagic homebrew game! Really cool to be able to do that with no hacks or trickery.