Here is a plugin that adds support for TODO comments in "Tasks" view.
It was created for Flex Builder 2 in 2006, but I tested, it also works for Flex Builder 3.
Here is link to the request on Adobe's JIRA: FlexBuilder should parse //TODO comments into the normal Task panel . I have voted for this issue.
However, Dirk obviously did a good job in his plugin, such that Adobe decided to defer and closed the issue!?