Here at TUAW, we're rather fond of XCom Global. Uncle Mike recently took a trip to the UK and brought along one of their easy-to-rent MiFi units. For about $18 a day, you gain access to local data, which is easily served over Wi-Fi to your iPhone or laptop.
The coverage areas and expected bandwidth vary. For example, you can expect 7.2 Mbps in the United Arab Emirates but only 1 Mbps in Guam. Check the coverage maps from the XCom site to make sure the areas you are visiting will allow service.
If you are traveling between countries, you can set up multiple unit rentals. You are still charged a daily rate beginning when your travel commences and ending when you return the US. The company's FAQ answers many common questions about their service.
While traveling, your data usage is unlimited. So what does "Unlimited" data mean? Check out XCom's "fair use" FAQ. Providers in various countries may cap usage or throttle bandwidth for users who draw heavily on data, but for normal computing needs (assuming you're not downloading several multi-gigabyte movies per day), you should be covered.
Rent for 7 days, and XCom throws in free shipping (normally $29.90 for 2Day FedEx; you will pay extra for Overnight regardless). If you are renting for over 14 days, the daily rate goes down to 16.15/day.
One thing you'll want to consider is adding in the option for the $2.50/day battery booster pack. MiFi works best when it's always on -- a battery pack offers a way to avoid switching it on and off (although it will get quite warm, be sure to keep it in a ventilated bag or pocket). Alternatively, you can supply your own battery pack.
Mike recommends the ZAGGsparq battery pack. This $100 device is international-aware, allowing you to charge from 220V sources; you don't need a voltage converter, a simple outlet adapter is fine. This will save you the rental fees and provide handy power on the go even when you're back at home (it's rated to recharge an iPhone four times over).
Also you may consider opting for the insurance. At $4/day, it covers up to three devices, allowing you to worry less about pickpockets and lost equipment. With insurance, you will pay a $160 deductible per unit rather than the $800 XCom normally charges. Three devices, $480, which is quite a bit less than the $2400 you're on board for.
Together all these charges do add up to a hefty $25/day or so, not counting possible shipping costs, but as a business expense, that's relatively little compared to what you'll be spending on flights, hotels, food and incidentals.
It's unquestionably a much better deal than roaming data. It also avoids the hassle of trying to jailbreak your phone and buy compatible SIMs while trying to travel at the same time.
Love and kissies,