Give a nod?
Little hand wave?
Keep your eyes locked forward and pretend they are not there?
Well, I have had mixed experiences with other runners, I generally try and give a little wave or a head nod. Some runners are very friends and give you a nice hello or a wave, but it seems like with the growing number of ipods out there now that there is a little bit of the social aspect that is lost out there. Below is a podcast form Endurance Planet that talks about this subject.
Enjoy!Endurance Planet Podcast