WP 2.6 allows sites to move the wp-content directory around, so plugin developers like me can’t depend on them being in a predictable location. We can look to the WP_CONTENT_DIR and WP_PLUGIN_DIR constants for answers, but a better solution is likely to use the X_url() functions. The most useful of those is likely to be plugins_url(). Even better, you can give these functions a relative path and they’ll return a fully qualified URL to the item.
WordPress 2.6 Plugin and wp-config.php Path Changes
Ozh’s tutorial explains the details, but the short story is that we’ll soon get WP_CONTENT_URL and WP_CONTENT_DIR constants. And this is more than just convenience, 2.6 allows site admins to put those directories anywhere they want, so the constants will be the only reliable way of finding that info.