It would be very useful to have a shared bib file across my research group which can be kept up-to-date by any member of the group. Are there any tools for this sort of thing? I know about Mendeley which allows bib sharing and syncing. Are there any other tools?
You should take a look at Mendeley (http://www.mendeley.com/welcome/), great free software. I'm not fully sure the function you're looking for is there, but there are elaborate options to share citation information with collegues (also in a hidden environment so only people you like can access :)
You could try CiteULike. I use it as my primary reference database; it makes it easy to add an article by automatically scraping journal webpages. It exports to BibTeX (very easily; you can bookmark a particular URL which will download the bib file). Moreover, it has a 'groups' feature where multiple people can share a common library.
We are simply using svn + JabRef + standardised JabRef settings; works well. JabRef automatically enforces a consistent formatting and ordering of the Bibtex entries. After saving the file in JabRef, you can easily use "svn diff" to see what was changed, and commit the changes if everything looks good.
The only issue is that JabRef tries to support collaboration as well, which doesn't make much sense if you are using svn. Therefore it seems to be easiest to close the file in JabRef before running "svn up" so that JabRef doesn't try to do anything clever.