Thank you for your kind answer.
Last night i printed the article from Technet magazine. I will soon have some more time to read it with more attention. I've just read the introduction and the approach seems very close to my idea.
I'm not sure that every requirement is matched by that solution but i will post a complete feedback asap.
I've installed SQL Server 2005 and tried many new features. The Service Broker was still... queued.
Unfortunately, i've seen Technet magazine advertised on MSDN Magazine but it is not available outside US. Now that i know its available online, i'll take a look at it at least one time each week.
I've almost finished a solution based on SQL Jobs and that article could have brought me to a different approach. I'll let you know about this when i'll reach the bottom of the article.
About Async calls : i'll search info about this, but i suppose that the application must be left open to wait for the call completion.
This does not match the requirements because the users want to have more freedom (must be possible to close the app and take away the laptop for external activities).