I've spent nearly a year in Ethiopia over various trips and it's an amazing place. Having seen it both ways, it's infinitely more rewarding living there, than just visiting as a tourist. Many visitors find the Ethiopians seem quite agressive, and it can take a bit of time to get past this. Such warm and welcoming people, though, when you do.
If you can head into the Omo valley while you're there, then take the opportunity. I found it a bit strange, 'cos essentially just going to look at people, but they genuinely enjoy interacting with visitors, if you take the time to, and it's just so different from anything else you'll see.
I'm pretty jealous of you going. Despite the time I've spent there, there's still so much I want to see and do. I've had a lot of dealings with these guys http://www.linkethiopia.org/index.html including doing this http://www.gondarlink.org.uk/bike/index.shtml for them a few years back.
Good luck and enjoy.