Hold on , there is one way!
The only way to downgrade 3.50 as of now is to buy the psjailbreak dongle for $99, this will downgrade 3.50 as many times as you want , so in theory you could buy it and downgrade 10+ of your friends ps3s aswell! charge them $10 each , $1 profit lol!
But if you can wait a few weeks maybe more there will probably be a free method, hopefully, nothings certain but I reckon psdowngrade will be copied and then all dongles, psbreak, ps3go, p3free etc will have the same abilty. I think minimus or other third party usb dongles are the way to go, at least they are open source software and so updates are quicker.