The only real news we have had to date was last summer when a UBI PR person (Jacint Erdei) at a trade show let slip in a
video interview (lower one on page has English Subtitles look at time period from about 04:22 onward ) that once development on Tom Clancy's HAWX was over the guys at Ubisoft Romania would be turning their hand to "the fifth installment in a very popular series" which we are 99% sure means SH5.
This very day, Ubisoft announced seven new titles to be released during 2009-2010, unfortunately for us SH5 was not amongst those disclosed, but there is still hope, keep your fingers arms and legs crossed!