For any 'in-transit' visitor thru the NBO airport it's the same as most every airport anywhere... no Visa for the country where waiting and never technically actually putting a foot in that country.... just hanging at the airport.
For those who only need a Transit Visa (Kenya USD$20) this is actually good for less than 48/hrs, if that... not 3/days.
For those visiting both Kenya and Tanzania, it's a single-entry visa for both...for those arriving Kenya can return after time spent in Tanzania... no multiple required.
Most important though - for entry to Tanzania from Kenya, you will require a Yellow Fever inoculation and proof of same at the TNZ airport or land border crossing. There's no guarantee border agent will ask to see this 'proof' but if they do and you don't have, they have the option to not allow you to enter or escort you to a separate area where the jab will be given and you pay USD$50.
And, don't worry about how long or short the wait for a Visa or if just an Immigration queue... you're in Africa, pole pole (slowly, slowly) - you're on Africa Time! Take a deep breath, better yet... remove your watch till you board your homebound flight, all has been timed/scheduled by your tour operator, so you don't need it.
Remember - "The white man has the watch, the African has the time!"
Edited: 6:07 pm, January 24, 2013