I'm happy to report that the new firmware (v2.03A) apparently fixes the fussiness with SDHC cards.
My 661 is the Oade SuperMod'ed version. I ran two tests with each card:
1) Digital-in at 24/96
2) XLR-in, P48 on, 24/96
I used the 32 GB PNY Optima and 32 GB Kingston, both class 4 cards (linked in a post above). As mentioned previously, both these cards failed using the old firmware. With the new firmware, in all 4 tests, the 661 recorded > 7h with no hiccups. I stopped the tests intentionally (and once unintentionally) after a suitable amount of time had been recorded.
Thanks to Marantz for finally dealing with this issue. To me, this was the achilles heel of the recorder. I loved everything about it, but I couldn't find anything besides my single PNY 8 GB card that I felt confident to record on. The ability to reliably record onto a variety of SDHC cards increases its utility 10x for me.
I'll definitely report back if I encounter any problems, and likewise I hope others report their success/failure with this new firmware, but for now I'm cautiously optimistic that they've fixed the problem.