Can you do a partial checkout with Subversion?

Indeed, thanks to the comments to my post here, it looks like sparse directories are the way to go. I believe the following should do it:

svn checkout --depth empty http://svnserver/trunk/proj
svn update --set-depth infinity proj/foo
svn update --set-depth infinity proj/bar
svn update --set-depth infinity proj/baz

Alternatively, --depth immediates instead of empty checks out files and directories in trunk/proj without their contents. That way you can see which directories exist in the repository.

As mentioned in @zigdon's answer, you can also do a non-recursive checkout. This is an older and less flexible way to achieve a similar effect:

svn checkout --non-recursive http://svnserver/trunk/proj
svn update trunk/foo
svn update trunk/bar
svn update trunk/baz

Subversion 1.5 introduces sparse checkouts which may be something you might find useful. From the documentation:

... sparse directories (or shallow checkouts) ... allows you to easily check out a working copy—or a portion of a working copy—more shallowly than full recursion, with the freedom to bring in previously ignored files and subdirectories at a later time.

Or do a non-recursive checkout of /trunk, then just do a manual update on the 3 directories you need.

If you already have the full local copy, you can remove unwanted sub folders by using --set-depth command.

svn update --set-depth=exclude www

See: http://blogs.collab.net/subversion/sparse-directories-now-with-exclusion

The set-depth command support multipile paths.

Updating the root local copy will not change the depth of the modified folder.

To restore the folder to being recusively checkingout, you could use --set-depth again with infinity param.

svn update --set-depth=infinity www

Sort of. As Bobby says:

svn co file:///.../trunk/foo file:///.../trunk/bar file:///.../trunk/hum

will get the folders, but you will get separate folders from a subversion perspective. You will have to go separate commits and updates on each subfolder.

I don't believe you can checkout a partial tree and then work with the partial tree as a single entity.

Not in any especially useful way, no. You can check out subtrees (as in Bobby Jack's suggestion), but then you lose the ability to update/commit them atomically; to do that, they need to be placed under their common parent, and as soon as you check out the common parent, you'll download everything under that parent. Non-recursive isn't a good option, because you want updates and commits to be recursive.