Today I had to add and SVN external project to my wordpress based project, nothing strange but the fact that the external project (NextGen Gallery) is hosted on Google Code and the authentication is different from my main repository.
After a bit of search I've found that you can specify the user for external resource by adding it before the domain with a @ so, for me the sintax was:
nextgen-gallery https://simone.fumagalli@nextgen-gallery.googlecode.com/svn/trunk/
Once you set up the SVN property give an update to the working folder and Eclipse will ask for the password.

SVN External with a different username in Eclipse