I use the Transcend 4GB SDHC class-6 card in the SD slot with no problem other then speed. I also had an older 4GB Sandisk Ultra II SDHC card, that was even slower. I kept that one in the camera, but almost never used it.
I use the SD slot as backup sending a small jpeg to the SDHC card, because if I send anything larger it becomes the buffer limit rather then my 8GB Extreme IV CF card. While I don't hit the buffer very often, it can get annoying at the wrong times.
According to Rob Galbraith's database SDHC cards are about 20% slower then CF cards in the 1DsMKIII, but are faster then CF cards on the 1DM3 (although about the same overall MB/s speed on both ). Still trying to figure that out, but his database matches my experience for all cards I have so I trust it to be reasonably accurate.
Short of getting a Ducati edition CF card, I have the best CF card for my camera (and I got it before he updated the list), but could use a better SDHC card, but until they have 8GB SDHC cards that run as fast as an Extreme IV CF card, I will stick with what I have.
Just my experience,