Michael McCracken has released a new version of BibDesk, a graphical BibTeX bibliography manager for OS X (see also a new version of Pybliographer, which uses Gnome). BibDesk allows drag-and-drop management of citations, produces an RSS feed of your bibliography to which others can subscribe (and should soon support reading other people's RSS feeds as well), and apparently works well with TeXShop for typesetting.
BibDesk should soon be able to search and import citations directly from PubMed (as I believe Pybliographer already can). In the meantime, I've made a few improvements to HubMed's BibTeX output that should make it a bit easier to work with.