
I've created a somewhat simple demo to get some first hand impressions. Every page loaded from this website results in a ball dropped from a platform on my land in SL. Every ball contains some information about the website visitor.
You could call it a very annoying Web Analytics program :o)

Real life businesses might consider having a representation within this virtual world. A lot of large corporations has already done so. To name a couple: IBM, Computerworld, Jyllandsposten, TV2 News and Roskilde Library.
Do it for marketing. To have something to talk about with your customers. Do it to get some knowledge.

SL runs on several servers owned by Linden. You can exchange information with the virtual world through a XMLRPC gateway. There are also some initiatives that provides high level languages such as java and csharp with a (complete) API.
Linden has also released the client as open source.