Well, I finally found a clue:
http://forum.ppcgeeks.com/showthread.php?t=9434
Looks like the older dll's worked up to 4gb and the new ones posted here handle larger amounts.
In fact, those with the 6gb card were only seeing shy of 2gb, JUST LIKE ME.
I upgraded to the new dll's praying the problem would be solved, but no dice--killed my SD access altogether. Putting the first batch of sdhc dll's got me back running, but something is up.
I'm hoping the Luv2Chill guy can help me out.