Originally Posted by Herky21
It'll taste like grass.
Although from my experience, if you're doing this dry-hopping while the keg is in the fridge (ie cold) you can go a lot longer without them giving grassy off-flavors. I let some sit in a keg of IPA for a few months and they never turned grassy, so long as you keep it cool. I like the idea of tying a string to it, even if you leave it long enough that its always submerged, it will make taking it out a lot easier.
Also, definitely get one of the mesh dip tube covers. I've heard a stainless scrubby will work, or I think Austin HB was selling some for fairly cheap a while back. I used whole hops and tossed them into a sanitized grain sack. It was alright for a while, but after about a month or so I was having to clean out the "out" poppit every week or so to prevent a bunch of clogging and foaming.