It turned out to be pleasingly trivial to
(I'm thinking of checking it out but it doesn't look like you can browse the store from a regular Web browser, and I can't find any information on accessing the PSN store from a PSP running custom firmware that's more recent than 2010.)
That's probably because you don't NEED any special programs to access the PSN store from CFW 6.60.
I upgraded to 6.60 and it was
pleasingly smooth: download and install Sony's firmware upgrade, download the latest
CFW archive, put PRO_UPDATE and FastRecovery in the GAME directory, run them in that order, and Bob's your uncle. You can activate your device and access the store, and yes all my homebrew still works too.
How smooth was this? So smooth that I have a thoughtful neighbor with an open access point so I didn't even have to bother setting up a router with WEP. (PSP 3000 doesn't support WPA2, and obviously I'm not going to use WEP on my main router.)
So, okay. Seeing as how I'm not going to buy FF4 again because I still haven't finished the LAST version of FF4 I bought, is Dissidia/Duodenum worth ten bucks?
...is it worth putting my credit card information on PSN?