Yes, it is true that you can be penalized for having multiple pages with the same content. There is not currently a way in DotNetNuke to prevent it from happening though.
My opinion is that is not a crtical problem though. It's one of those things that should not be the most import thing an SEO expert should be concerned with. Sure, it should be addressed, and it would be nice if you didn't have to worry about it at all, but there are much bigger ways to use a dynamic content management system to benefit SEO.
The only way a serach engine is going to find these "duplicate" links is if they find a link somewhere else that is built like that, so the probability goes down of having the duplicate content indexed.
If you are really concerned with it though, you can turn off the friendly urls in DotNetNuke.
Contrary to pouplar belief, the major search engines do index urls with querystring variables. It is just that you can't get a lot of benefit out of tabid=36 as a keyword.
This is a problem with the login, register, terms of service, and privacy information though, because your own site will generate a different link to each one of these pages for every page that you put in your portal.
I think there is already a bug logged for that, but Scott McCulloch and a few others have come up with good workarounds for that.
BTW, something that is related. Your Terms and Privacy pages are actually stored in memory and are very large, so a good way to kill two birds with one stone is to actually replace the internal links for those with links to static html pages.