I travel around a lot for work as well as when I escape from the job, and so although I don't have experience of the Americas I've had a reasonable experience of trying to connect in Africa, Middle East & Central / SE Asia.
Your best bet is to go for a mix of internet options - don't try to find one thing which will give you a global solution. A Thuraya with a data cable (or Bluetooth with the newer ones I believe) will get you e-mail at least, and coverage is good (except the Americas!).
There is a Thuraya modem driver for Mac OSX available online - I think it's the Motorola Sprint driver - if anyone can't find it & needs a copy PM me
Unfortunately the Iridium phones have had the InfraRed modem removed in later models (the window is still there but nothing behind it) so you only have the bulky cable option there, but they will work almost everywhere (but VERY slow!)
If you want to guarantee web access everywhere except the poles then the various BGAN systems from Inmarsat are the way to go, but not cheap... Don't be tempted by an 'R'BGAN from e-bay - they just turned off the system so they are just plastic paperweights now!
But - the 3G USB data modems are really coming into their own now - I'm in Sudan at the moment using a Zain one (with terrible network coverage, but hey, it works as long as you are prepared to keep hitting redial until you get a connection) - try to find a network which spans several countries you plan to visit and you may be able to use the same SIM card in all of them. Also check online when you have a chance and see what they show on the coverage maps - and remember - you might get a signal from the top of the hill even if nothing in the valley!
Just using your normal phone as a modem through a cable or Bluetooth is also a good option - surprising how many networks support GPRS / 3G data. I have a Vodafone SIM which works in most countries and will let me get my e-mail (at a price for the roaming!) if nothing else works.
Then there are Internet cafes - and of course you can always try to find unsecured WiFi networks & hop onto them - get a WiFi detector gadget rather than trying to cruise through towns with a laptop open in front of you...
Also, if you know someone with a connection to a single computer you can set that machine to share it - so someone with a laptop connected by a USB modem can share their connection via WiFi (Win XP and onwards, plus all 'proper' operating systems as well) so even if you can't use their connection direct you can piggyback on their micro-network.
So, no one solution - lots of partial fixes. Get to know how to swap around between network settings (Macs are much better at this if you are looking for a laptop to take, but getting drivers for 3G USB modems is a bit trickier - but mostly possible)
Carry ethernet cables and a small WiFi hub if you have space - it makes it much easier to hook into a network if the opportunity comes up, and learn all about the various networking options on your computer before leaving home...
Don't expect broadband, but if you just need to check your e-mail every couple of days then it's usually possible, with patience & an ability to bodge. But then you wouldn't be reading the HUBB if you weren't like that!