Many internet providers also provide Wi-Fi service for an extra cost. Usually you can get a flat rate deal to use as much as you want, but prices vary dramatically. It's still a fairly new market so there are still vendors trying to get a price premium off the unwary. Look around. You don't have to get Wi-Fi from whoever you're getting your net access from now.
There are also services like Boingo and T-Mobile that provide Wi-Fi service. What they do is specialize in the Wi-Fi that regular ISP's are now starting to add into their service line. They buy up the rights to Wi-Fi in certain buildings, or help establish a Wi-Fi presence there in coordination with building and store management, and bundle that coverage building by building into a Wi-Fi presence in a city that they then send to you. Just like with the big vendors, they compete with each other to get the juicy spots, and coverage is quirky. This is because Wi-Fi coverage is very short-range. You might get it to work fine in a building, but the quality likely will rapidly drop off even in a single building, and once you talk about covering the block, you're pretty much out of luck. And there are millions of blocks in some cities.
These services charge for access by the hour, by the day, or have a flat fee for unlimited useage per month. Monthly fees generally go from 20 to 30-ish bucks, and daily use can be as high as 10; hourly can be much more expensive still.
Check out jwrie.com and
www.wifinder.com for a listing of the major wi-fi providers, along with some reviews on coverage at certain spots, etc. You'll see that some spots in your city that you would expect to be well-covered probably aren't -- everything's still spotty at this early stage in almost every American city.
Some spots reliably have coverage, because they've made a deal with certain providers, and some enterprises find it necessary to provide their customers with Wi-Fi, like colleges and hotels. For instance, Marriott hotels are all wired for Wi-Fi and Starbucks coffee shops have a deal with T-Mobile, so you can always go there and pay up a big rate for the day or hour, or just get a T-Mobile monthly contract if it's worth it for you.
It's a bit of a pain and complicated, because it's like trying to make a phone call not like we do it now, but as if there were dozens of phone companies and you had to pay all of them if you wanted to just walk up to a phone booth anywhere and use it. And, there's not even that much coverage in many places.
It's pretty chaotic now, so the freedom of roaming around with your laptop and using Wi-Fi is actually pretty limited, but with any luck you'll be in a city where that's not a problem. Some towns and shopping centers are even starting to put in free or subscription wi-fi covering broad areas, so wi-fi penetration is slowly getting less expensive and more common. The increasing competition and decreasing cost of setting up new wi-fi networks is helping make things more consumer-friendly, but for many people it won't be easy to find wi-fi for a reasonable rate in all the places they want to go for quite a while to come. The fantasy of easily being able to go play online poker on the beach or anywhere you'd like is still a very distant prospect.