I have a strange similar problem using i2c with the overo.
I have ivero earth + summit + sensor SRF08. I use this level converter http://www.sparkfun.com/commerce/product_info.php?products_id=8745
Everything seems to be ok for SCL but SDA is not working at all. I try at 400khz, 100khz but when I try i2cdetect or i2cset SDA is always UP.
Thanks for your answer !