Here it is in short:
$ cd /dir/of/your-branchWhere ^/trunk is the url of the branch you want to merge from. For more details, have a look in the svn book basic merging section.
$ svn merge ^/trunk
Also this article on svn merging explains it fairly well.
The svn 1.6 release notes and 1.5 release notes also talk about various svn updates including merge enhancements amongst other goodies (like improved python bindings).
Really, merging is not too bad in subversion now. I know this post won't stop everyone from using 2004 reasons arguing over version control systems... but what ever.
Ok, bzr, hg, and git all have lots of nice features - but merging in svn is pretty easy now so please bash(zsh) it for other reasons if you must.
Now, let's move back to the vi VS emacs argument ;)