How do I setup a wide area wireless network?

I manage the network for my parents small business (an after hours type thing just to keep them from having to pay someone), and I would like to setup a wireless network for them. They have two large warehouse type buildings with a small private "road" in between. I would like them to be able to have wireless access anywhere in or around these buildings... how is this accomplished? I know I'll need a few access points, but how do you bring it all together so that you have "one" large wireless network?

Also, if they add another location somewhere, is there anything that can be done to have a wireless network there that "matches" the one at the first location? Basically so Windows would recognize it all as the same network and know to connect when in range of either?

Cisco Wireless controller with a bunch of light-weight Cisco Access points (running LWAPP) will do that. The Controller is connected to the access points using cable primarily (but can also hop using the actual wireless if needed) and you set all the configuration in the controller - the access points just obey automatically and cooperate in making client roaming as seamless as possible without any special drivers needed on the clients.

This is of course expensive, but the concept is the same for all major brands and I bet some like HP or perhaps even Netgear has similar solutions in store.

Most likely there are some open source alternatives perhaps with custom firmware for some cheap consumer access point that can accomplish something similar with a central server - but I haven't researched that.

A central controller unit and a thinner kind of access point with not much logic except for the actual radio in them - connected to the controller by whatever means available - which will coordinate access, configuration, roaming and so forth. You can connect one of these light-weight access points anywhere in the world as long as it can talk to a linked controller - it will (if you want) appear as the same network.

There's a nice solution hostapd which implements Inter Access Point Protocol (IAPP). That will allow you to roam from one access point to another seamlessly. If you can arrange overlapping coverage of APs it'll appear to be one big hotspot. There's a nice introductory article covering it.