If you use Dreamwidth's some_user_name tags in your post, the links will go to some-user-name.dreamwidth.org — even on the LJ post.

Is true, and what's also nice is that if you use the site= attribute in the user tag, it translates when you crosspost. Frex, if you post in Dreamwidth about some_user_name site=livejournal.com , it shows up on your LJ post as some_user_name.livejournal.com