There are a handful of ways to dry hop in kegs and be able to separate them from the beer when you want to. I use one of these: http://www.stainlessbrewing.com/Dry-Hop … p_155.html
Another way is to put a stainless braid over the intake end of the liquid dip tube, then push the beer to another sanitized keg when done dry hopping.
I have only left hops in the keg for an extended period once and I felt like it tasted more grassy than normal. I was surprised, because I heard several guys say they don’t have that problem. This was also a recipe that I’ve made a few times, so I know what “normal” tastes like.