I'm actually finishing a review on the mobile broadband service I went with for my site, but the gist of it is - there are a lot of choices. Most of the big boys, like AT&T, Sprint, Verizon, etc... are expensive and lock you into contracts. Virgin has contract-free plans, but they don't offer much data usage. Their unlimited 3G plan throttles you after you go over 2.5gb/month - which you hit quickly uploading and downloading big photos.
Many require you to purchase the device as well and pay an activation fee. I ended up going with Millenicom who is a reseller of mobile broadband - they buy up bandwidth in bulk from verizon and sprint and resell it to their customers for a lot less than either of those networks do. No contract, but there is an activation fee and for many of their plans (the byod plan lets you use your own device) you have to buy the device. I went with the 20GB/month plan which uses verizon's network and I'm really happy with it. $60 a month with no contract. I use it as my main internet and can get online in seconds just about everywhere in the US. The speeds in many places are on par with DSL.
If you just need wifi on the go from time to time, McDonald's, Starbucks, Barnes & Noble, Dunkin Donuts and just about every other place like this now has free wifi. If you don't need it all the time, I would go the McD route and just camp out there for an hour and get your work done.