Sony recently registered a trademark for "Resistance 2". The first Resistance (Fall of Man) game was a pretty good launch title for the PlayStation 3, so there is really no reason why Sony wouldn't make a sequel. However, until this trademark registration, Sony hasn't said a word about a new game in the series. Although this doesn't seal the deal that another game is definitely on the way, it makes it much more likely. If Sony does make an announcement about a new game in the series, we will be sure to post about it!
Here is the link to the trademark document: