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.