My Ableize site has just had a PR update from 3 to 4 and most of the sub categories have also gained a good healthy PR most from 0 to 3 which is great and pleasing considering all the work I have put in getting links etc. However, 4 top level cats and subcats have NO PR and I just realised the reason why. I recently had a shuffle round and changed a few cats for example one cat called Access and Travel at http://www.ableize.com/access-and-travel/ was previously called Access and Accommodation. I split the cat up to create two cats, because of this they were looked at by the likes of Google as new and therefore had no history or links back. So my advice when creating a directory especially a niche directory is spend a great deal of time on your category creating so you can avoid change at a later date. Any changes to top level cats will have an overall effect on not only the cats changed but also the entire site due to the internal link structure changing and of course the loss of inbound links if pointing at anything other than your home page.