I’ve written about SDHC several (1, 2, 3, 4, 5) times. In a nutshell it is an updated version of the SD standard that will let you use between 4GB and 32GB of storage.
The current SD spec allows only 2GB. You can get a standard 2GB SD card for less than $20, miniSD will set you back the same amount, microSD is a little more expensive at $36.
Symbian Freak just wrote an article explaining what exactly SDHC is.
I’m really hoping we see that logo on a retail Nokia package this year.