This is what is commonly known as "hotspotting". In a nutshell, a person uses a laptop with wireless capabilities to use a wireless hotspot to access the Internet. This is commonly used in places such as cafes, which provide wireless Internet access. You could also sniff around and find a hotspot elsewhere outside of cafes by jumping on someone's unsecured wireless. I should note that hotspotting is potentially dangerous, as it can be used and exploited for malicious intent as anyone can basically read data such as personal information and/or passwords which are transmitted unencrypted, such as through web browsing. This is even more dangerous when using ad-hoc networks. It is also, in some cases, illegal.
It is not quite possible yet to have proper and stable "roving" wireless access, such as while driving on the move. There is technology being developed which may eventually solve this problem, such as with WiMAX, however it has yet to be widely deployed.
Addendum: People can (and do) set up "rogue" hotspots with the intention of stealing information and/or to break into your PC. When hotspotting it is vital to be on the lookout for such things. I personally like to use
AirDefense Personal as a set of extra eyes and ears on the lookout for ad-hoc networks (bad idea to use those, you want to use dedicated APs), wireless phishing, and so forth.