I've worked with the USRP and USRP2 extensively trying to use the hardware to detect cosmic rays using forward scatter of radio waves. Both are incredibly fun, versatile tools, especially if you learn a little bit of verilog.
I too have been working with the USRP lately for doing underwater freespace communication - basically shooting lasers through water to communicate. It's a very neat system and I was able to get a streaming video system up and running within 30 minutes. Very cool.