It's great when they do that! My Sabu is not the msot affectionate of cats, although he greets me with great exuberance with big head butts, but he is always trotting around with his tail sticking right up in the air! It's great to know he's happy
My friend's cat Tigs does the same, tail is always up, but you can't blame him really - he has the most magnificent squirrel tail! It's quite funny when you're indoors and Tigs is wandering about, as sometimes all you can see is the top of his tail moving between the furniture, just like a periscope!