One question, are these memory cards Sony branded?

Yep.

If the card has become corrupted it is very possible the PS2 is hanging when it tries to read the card.

The card reads fine once the PS2 has loaded the game. I've also tried it with a brand-new card (so you'd expect no corruption), and it won't format the new card (which, even if the card was corrupted, you'd expect to completely wipe the card).

Looks like it's hosed. I'll see if I can find a game store that does repairs next time I'm in town, otherwise it's time for a new one.