Phil Langdale has patched the 4.2007.26-8 kernel with the full set of SDHC patches, some of which Nokia left out from the official release. The kernel is available here.

So far so good (with dual 4GB Transcend Class 2 SDHC cards). A quick "read" test with a 4GB Transcend C2 card in the external slot reveals a speed of 12.1MB/s (234.375MB read in 19.31s).