I have a small question of the joomla license. I know Joomla is release under the GNU/GPL license and that it means I can freely use, copy and alter the source code as long as the license information is retained. I know that this is possible for non-commercial use, but what about commercial use ?
The reason why I ask this is because I've been asked to create a website for a store (a bio-store to be exactly) where the owners can place announcements from time to time. Can I do this with joomla ? (btw. they have no intentions for having an online shop, just some announcements and some information about the store).

No problem!
There are a number of commercial sites that use Joomla as their CMS. There is even a version of Joomla that has a shopping cart and related templates pre-installed.

You may also note that there are a number of commercial licensed products for Joomla. I believe that under GNU/GPL, you can even charge for the download and installation of Joomla. You can't charge for the software but you can charge for services you provide regarding the acquisition and implementation of the software.

Of course, when in doubt, you can always send a formal inquiry to Open Source Matters or check out this information page about Joomla! License Guidelines.

Hope this helps,
vujsa