I'm still stuck at 3.55 firmware till ReactPSN 2.0 supports 4.21. Until now, reports says that reactpsn still doesnt work on 4.21CFW.
But word for the PSN access in 4.21. it's your chance to use PSN now and download stuff that requires activation. you will need this to generate your own act.dat, so you will have full support for reactpsn 2.0 later on when it's done.
To get it done: Just download any free DLC that requires activation, like the Uncharted 2 free DLC.