For those of you wanting VISA information.....we just returned from Kenya and Tanzania about one week ago.
On arrival in Nairobi we easily obtained our visa at the airport.....In our case, we obtained a 3 day transit visa for $20, because we were traveling to Tanzania several days later. Although we were doing a 13 day Safari in Tanzania, it was much cheaper to fly into Nairobi rather than Kilimanjaro. From Nairobi, it is easy to take a shuttle bus to Arusha ($25 one way) which is about a 4 hour drive. At the Tanzania border (Namanga), a multiple entry visa is easily obtained for $100US. Wait times vary, but in our case about 15 minutes going down, and 1 hour coming back to Nairobi. On the return we obtained a single entry visa for $50.
Many tour companies recommend getting visas in advance, but for us it was easier and cheaper to obtain them on arrival.