Check with the CDC (link) for immunizations, etc. They are very serious about Yellow Fever, so if you think they might worry about you, get your shot at home. I don't recommend getting the inoculation at the airport in Dar.
Visa is easy to get at the airport in Dar es Salaam for $100US cash. They seem to be taking more kinds of currency than before, but I still used a crisp $100 bill for no problems. Getting a visa ahead of time would speed travel through the airport.
Vodacom is used throughout the country, and I had good luck with connection strength and speed while there. I caused myself problems by messing with the settings while we had a brief outage, and when service was restored I was still blocked. It took some time for me to remember the settings that had been made when I got the phone, and to reset it to that. Note that although Vodacom is a widespread carrier in southern Africa and elsewhere, you can't necessarily roam from country to country. It's best to get a new SIM in each country.
