Can iPad be the screen of Mac mini?

Check out an app called "iTeleport". I know it works for iPhone, pretty sure there would be an iPad version as well, but you'll have to check.

You can use Air Display to use the ipad as a secondary monitor;

http://avatron.com/apps/air-display

I doubt you can use it as a primary monitor but you might ask in their forums. I don't think the ipad is probably suitable for this, myself, but ask the people who know for sure. It may be better with the iPad 2.

I would love this feature also! I'd ideally like to be able to treat an ipad (which I dont' have yet) as a monitor for a mini - kill two birds with one stone in a sense. It would be a laptop & an ipad all in one.