I thought I'd share this as we needed to do it and I couldn't see anybody else who had done it.
Our problem was that we had edited some files on a remote production machine rather than on the development machine and then pushed out to the production machine, Yes we know we shouldn't do that, but when things get a little hairy, sometimes one has to do things we don't like
We then had to work out the changes on the production machine, understand them and merge them back into the development code branch.
We could transfer the whole directory over but we thought we'd see if we could do this using diffmerge.
1. There's no obvious way to do this in diffmerge by itself. If there is we missed it.
2. We looked at alternative competitors to diffmerge who claimed to do it. It appears that they don't have a built in FTP client to do it but rely on the Finder and the ability of Mac OS X to load up an machine via FTP direct to the finder. Ah! the penny drops.
3. Connect to your remote machine using the Finder and Apple-K. Use an FTP URL to connect to the remote machine You'll need the FTP logon and password of the remote machine clearly.
4. Once you've loaded up the FTP machine in the finder, you can easily use diffmerge to compare files as the FTP machine is available via /Volumes/<FTPMachineName>.
5. Errr.... profit
In hindsight we should have known how to do this but had forgotten about the ability of Mac OS X to mount FTP machines. Once we remembered that, the rest was easy.
Hope this helps somebody.
Rob