piatkow - 10:06 am on Sep 20, 2010 (gmt 0)
I used to maintain a directory site for a regional arts organisation. The home page had separate links for artists, venues and suppliers. People searching generically for venues in the region would land on the home page and regularly go away rather than click the link for venues.
For good SEO you want links at topic level. While it doesn't technically matter if these are to domains, sub domains or pages within a single master domain I suspect that other webmasters will be more comfortable linking to distinct domains.