If you use Aperture and you like to organise your keywords hierarchically using the Keywords HUD, then you may find this page at Bagelturf useful, especially the section about moving keywords to the top level. I couldn’t work out how to get keywords which were in folders back to the top level – it turned out to be because I had too many keywords visible. This hint gives you a workaround, and it’s generally useful to remember that search box at the top.
My list of keywords was getting quite long, and I often had duplicates at various places in the hierarchy – ‘Seattle’ came under ‘USA > Washington State’, for example, but it also came under ‘iPhoto’ because many of my photos were originally tagged there. Typing the first few letters of ‘Seattle’ into the search box allowed me to see both and merge them easily.
This is the solution to another problem, by the way – that of getting the same keyword twice at the same level with different capitalisation.
Drag the one you want to change into a different level of the hierarchy – for example into a temporary folder. Then rename it to the right capitalisation, and drag it back to where you want it, using the search box to make life easier if necessary. Aperture will ask if you want to merge the two keywords.
You don’t even need to create a temporary folder, in fact, you can drag the one you want to change inside the correct one – if that doesn’t confuse you – rename the inner one to match, and then drag it to the level above to merge with its parent.
Hope that’s useful for someone!