DotNetNuke uses BSD as it's licence (see http://en.wikipedia.org/wiki/BSD_license for a description), which is a commercial friendly licence i.e. it's not a viral opensource licence such as GPL where new code "inherits" the licence. .In real terms it means you can do what you like as long as you retain the dotnetnuke comment blocks at the top of each page i.e you don't have to tell your customers that you're using an opensource project, and you can make any changes/renames etc you desire without having to contribute them back or credit them.

"DotNetNuke is provided as open-source software, licensed under a BSD agreement.  In general, this license grants the general public permission to obtain the software free-of-charge.  It also allows individuals to do whatever they wish with the application framework, both commercially and non-commercially, with the simple requirement of giving credit back to the DotNetNuke project community. "