You have no control over the wear leveling, SDHC (along with most, if not all, consumer memory cards) does it in the card. Whether this is good or bad is debatable, but you can not influence it. Adding a filesystem on top that implements another layer of effective wear levelling is difficult as that will sit on a moving target and the card vendors have each their own, closely guarded, wear leveling implementations.

ext3 serves me well on all my flash media and before I worry about wearing a medium out (a couple years at least) I generally have bought bigger media or the gadget it was used in got replaced/broke