Now i'm thinking... if it did go directly beneath the main text editing window, without separators or anything, how cool would it be if the text input box was the same colour as the background? So all you see is 'Tags:' just hanging there beneath the box...

While I understand how you feel about it, I hope this doesn't get implemented; it's *very* inconsistent UI-wise and it would cause a lot of people to go "...how do I enter my tags? ;_;", due to not being as accessible.

Generally, the best UI is one that people are familiar with. If you can type into it, it should *look* like a text box, or else nobody will know what to do with it.

(on a completely different note, your icon is entrancing! What is it, exactly? It's awesome, whatever it is.)