Hi,
This is already fixed in mojoPortal in svn and will therefore be in the coming release. It never was a bug on Windows, so it really wasn't a mojoPortal bug but I did fix this so it works on Mono.
All of the controls are registered in web.config instead of in the .ascx or .aspx. I fixed this bug by changing the namespace on the DataCalendar. It was using the mp prefix but came from a different namespace and this caused Mono to fail to see the other namespace using that prefix. I moved DataCalendar into mojoPortal.Web.Controls and that fixed it.
I'm not sure exactly how soon the new release will be but I think within 2 weeks. You can get it now from svn branches/2.x and build with MonoDevelop 0.12 or newer. If you have the svn client installed, you can get the code using
svn co https://forgesvn1.novell.com/svn/mojoportal/branches/2.x
Cheers,
Joe