As I'm currently upgrading my own block from 1.9 to 2.0 I have come across what looks like a bug in the way the online help is displayed in Moodle 2.0. Here are my findings.
1. In the "standard" blocks in moodle 1.9 the only block to have (one) help button is glossary_random. Help for How a new entry is chosen.
2. In Moodle 2.0 that Help button has disappeared.
3. In Moodle 2.0 it is possible to place a $mform->addHelpButton() button in a block's edit_form.php script.
4. However, when that button is clicked, instead of displaying the Moodle 2.0 new Help pop-up window, the help text is displayed in the current window. I expect that is a bug, and developers have "forgotten" the possibility to have Help buttons in blocks.