I have noticed a little change from SharePoint 2007 in SP2010 as I have been playing with the master pages. If you create a Team Site by default you can no longer change the masterpage from 'Site Settings' / 'Look and Feel'. You must first now enable the Site Collection feature:
"SharePoint Server Publishing Infrastructure"
and the Site Feature:
"SharePoint Server Publishing"
To do this go to 'Site Settings' under 'Site Actions':
If you are at the Site Collection Level click on "Site Collection Features" under "Site Collection Administration":
Click on 'Activate' next to 'SharePoint Server Publishing Infrastructure':
Back in 'Site Settings' go to 'Manage Site Features' under the 'Site Actions' section:
Click on 'Activate' next to the 'SharePoint Server Publishing' feature:
You can now change your master page in site settings under look and feel: