What will be best way to authorize, log and bill internet usage for
clients using wi-fi adapters? This solution is needed for hotel usage,
guests will arrive with notebooks with wi-fi adapters and we need most
efficient solution for authorizing and billing internet usage. We will
need internet session output to MS Access or MSSQL database...

Charing for Wi-Fi at hotels just makes business travelers angry:
http://www.nytimes.com/2005/10/11/business/11road.html

I'm with the other commenter on this. If I was staying at your hotel
and you nickel and dimed me like this, I'd never return. In fact, with
so many hotels offering free WiFi, I'd never even stay at your place
to begin with.

However, to make a note on one product you might consider, look at this:
http://www.publicip.net/

It's really well made. It does what you want. There is no output to
access, but you do get a report based on a number of criteria.
http://www.publicip.net/zonecd/features.php