I went to Kenya on Safari a couple of years back and had a great time! You'll love it!
The thing that really struck me was how some animals could be very close as well as far away. Some of the landscapes were also beautiful and required decent wide angle coverage.
So for me the most useful lens was one with a superzoom range. I can't believe Thomas hasn't already tried to twist your arm into it (
) but a DSLR with something with the range of an 18-200mm is perfect.
I was using a Canon 350D on that trip and was able to borrow a 28-300mm which was more useful than, say, a 100-400. These are seriously expensive lenses, but the ranges were what surprised me. I assumed the 100-400 would be better in that environment, but the one with the wider range was definitely more useful as a single lens solution.
That said, a twin lens DSLR kit with an 18-55 and a 70-300 would still be great...
As Thomas says though, the dust can be an issue when changing lenses. So I'd personally be tempted by a DSLR with a single superzoom lens or a superzoom compact. The new Sony H9 has a long range, stabilisation and focuses very quickly, which can be great for wildlife - look out for a review of it here soon..
PS - as for the E-400, remember the E-410 is coming real soon.