Visualising encounter data is fairly straightforward with Gephi. The trick is to convert your data into a GEXF file.

To do this I found a nice little script written for R on Cornelius Puschmann's Blog. This script (with very minor editing) takes a file in the following format:

from to start end
deviceA deviceB 1.00 2.00
deviceA deviceC 2.00 3.00
deviceB deviceC 2.00 3.00
deviceC deviceA 10.00 22.00
etc.

To understand this format imagine that each row reads as "devices in the to column are capable of receiving messages from devices in the from column between the start and end times".

GEXF files for some reality mining datasets

Unfortunately I accidently deleted the demo video from Youtube, but here are the datasets that I've already converted to GEXF format, I'll add more when they've finished processing: