You do not specify login credentials. Subversion has no way to force credentials, the server has to challenge you for them. I do not use SourceForge, but I would guess they have a different URL for write operations. svn:// protocol is unencrypted so I doubt they allow commit via that protocol. Most likely they require https:// or svn+ssh:// protocol to commit changes. Those protocols probably challenge you.
When Subversion is challenged for credentials, it fires a callback that Subclipse provides that will in turn prompt you in the UI.