The SDHC patches first became available in early January but weren't accepted into the kernel mainline until mid to late January (around 18 Jan, possibly in 2.18.21). Additional patches in early Feb added improved MMC support and may have fixed any SDHC bugs in the original patch (the patch developer didn't even have a N800 until early February!)

Nokia may have taken the view these changes were too cutting edge and decided they weren't worth the risk of delaying the release. Or not.

We won't know for sure until the next firmware release - hopefully SDHC will be baked in, but I won't be surprised if it's not.