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Just got back from Uganda and no issues there with the iPad. One provider to consider is Orange and there are tons of places to buy SIMs from. Not all may have the micro but they will cut it for you if need be.
In Brazil check for sim cards from either TIM or CLARO carriers. They got pre-paid 3G access for 0,50 cents a day. Just don't tell them it's for an iPad for they might refuse selling it and try to convince you to subscripe some of their data plans (which are a rip off!!).