There are more and more 8GB Class 6 SDHC available, but their maximum write speed is about 10-12MB/s in most cases. The fastest regular SD cards were able to write at 16-18MB/s. Faster 4GB SDHC are announced, but not yet available.
Only very few miniSDHC are available, and although the first 8GB microSDHC are already announced, my order for a 4GB microSDHC is seriously delayed.
I started updating the SD Compatibility page, the compatibility results for the 8GB SDHC will be added soon.