I'm looking to backpack eastern and southern Africa starting June 2013 for about 3 months. I'd like to fly to Addis, then work my way south to Cape Town zigzagging along the way.
What do I need to do as far as pre-trip planning? Visas, vaccinations, equipment, budgeting, accessing money on the road, etc? I would very much appreciate if you could refer me to someone with experience in this part of the world. Thanks a lot!

Look at www.lonelyplanet.com. Buy the guidebooks or borrow from a library
3 months is not enough time for backpacking Addis to the Cape
Have you been in 3rd work countries or just the US ?

I did 6 months in Africa just over 20 years ago now, agree - don't try and pack too much in. I did East Africa (Kenya, Tanzania (including Zanzibar), Rwanda, the then Zaire, back to Kenya, Malawi, Zimbabwe, Zambia, Botswana and SA. That took 3 1/2, nearly 4 months. You need to allow plenty of time, things don't always move that fast with transportation but then thats part of the char.

While in some places LP has become a joke, I think in Africa is still pretty good. Absolutely read the sections on vaccinations and visas in depth but some things you just can't plan for, we did find we had to change our plans sometimes. Tripadvisor is a pretty good resource too, alongs with the Lonely Planet blogs.

Good luck. You'll love your time, it's still the one trip in my life that stands out and I've had a few now (and some much more luxurious too but this is the one I remember most fondly).