I installed the SDHC drivers from xda-developers on my 6700 w/ Helmi's WM5 1.0.4 custom ROM. So far, so good, but I only have a 2gb card. At least it didn't screw up my phone.
It replaces sdbus.dll and sdmemory.dll in \windows. Back them up before applying the .cab.
Available in this thread courtesy of vboyz:
http://forum.ppcgeeks.com/showthread.php?t=9104